DirectX has an odd history. It was incrementally updated up to version 9.0c, so at this point all previous versions were encompassed within it. Then came DirectX 10 which was separate. Vista came with DirectX 9.0l (for basic compatibility) and DirectX 10. At this point it was advised to install DirectX 9.0c to fill in the gaps of 9.0l. So yes, at this point you were then running two DirectX installations and games would use the appropriate libraries. The same happened with DirectX 11, leading to three DirectX installations. I've not looked into DirectX 12, but the Wikipedia DirectX page does list sections for 9, 10, 11 and 12, so I wouldn't be surprised if that is also another additional set of libraries.

If GOG start offering CD images or original CD files, then they would have to be unsupported and provided as-is for users to play with. But would the game developers and publishers let GOG distribute these?

And again, why should GOG be expected to test on Windows Vista, XP, and older when these are unsupported OSes? There are so many different hardware combinations, it would be easy to end up with 30 test machines covering Win7, Win8, Win10, Ubuntu and OSX. Windows 7 has less than a year left, too, ending 14th January 2020.

And on your final point, if GOG start implementing an installer script that provides patches/fixes for specific OSes, and you include Vista, XP and earlier in that, then GOG should still be testing that this works with those OSes otherwise there is a risk that something goes wrong and the installer doesn't work as intended, and as has been said many times, why should GOG do this for unsupported OSes? It's additional work and so an additional overhead.

Horses for courses - you may like Win10 with all the choice it offers in Window themes (any colour you want as long as its white!), effectively compulsory spyware (you can disable user telemetry but some updates will silently re-enable it), on-screen advertising ("Use Edge now - use Edge NOW!!!"), watching those swirling circles for 10-15 seconds whenever you do something significant like launch a program, The Interface Formerly Known As Metro, Anniversary and Creator's updates breaking some of your programs, compulsory updates BSODing your system...

...but I'd rather stick with an OS where I can control everything that happens (thanks more to 3rd party security software that wouldn't be able to work in the presence of Kernel Patchguard), that I can customise (again, thanks to 3rd party XPLite and nLite - NTLite may now offer similar facilities on Win10 but requires online activation making it a no-go for me) and which I can install and configure completely off-line if I choose (again, thanks to 3rd party WPAKill disabling Windows Activation).

Seriously, I'd suggest Linux may be a better choice - Mac OS has good design features but Apple are the masters in user control freakery (when a project developed software allowing OS9 users to skin their desktop, Apple tried to shut them down while MS supported its Windows equivalent). Linux's main failing is the lack of application-level firewall software (by which I mean firewalls that can control network traffic on a per-process basis, e.g. allowing Konqueror to access ports 80 and 443 but blocking Firefox or any other software from doing so) but I suspect you may not consider that a make-or-break issue.
For the same reason that GOG are supporting games that have been done and dusted years or decades ago. Because people want them to and are willing to pay for that service. I have over 700 games bought when XP was still an officially supported OS and if any stop working due to "updates", I expect (reasonably, I feel) GOG (or the publisher, for those in active development) to fix them.

Now getting back on-topic, the game in question is not Vista+ or DirectX10+ but dates from Win 9x and seems very likely to run unaltered under WinXP, with far less demanding system specifications than listed. So the original question seems pertinent - why exaggerate the specs and artificially restrict the OS support (and thereby the customer base)?

GOG's service is to sell games, not get them working (in pretty much every case, all the work there is done by third parties - DOSBox, ScummVM, modders, no-CD writers, etc). Now if a game can only run under Win95, how do you think GOG would package it? A CVS version of DOSBox (supporting plug-and-play) with some type of Win95 image - assuming that MS agreed to license it (yes, watch out for flying pigs I know, but in this highly theoretical example GOG would be "supporting" Win95 in the same way that they are "supporting" DOSBox).

The fact that MS has dropped support for WinXP is, in itself, no justification for individual developers to leave their customers in the lurch. We are talking here about a game produced in 1996 which undoubtedly requires more work to get running in Win10 than WinXP. GOG's position here is purely one of convenience, not practicality.

From the https://www.gog.com/about_gog page.
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Upgrading classics for present-day
Even if the game is older than you are, we test it thoroughly, fix all the bugs, and apply patches so it runs flawlessly on your next-gen PC and on modern OSs.
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And from the https://support.gog.com/hc/en-us/articles/115000487189-Money-Back-Guarantee-Policy page:
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13. MY SYSTEM DOESN'T MEET THE MINIMAL REQUIREMENTS SPECIFIED ON PRODUCT PAGE, CAN I GET A REFUND?

Well, there's a reason why we post the system requirements on each game's page: so you can see for yourself what you'll need to have on your rig in order to be able to run the game. If your game doesn't work because you misread our system specs, all we can really offer is that we're sorry for you. :( Please note that we also cannot provide support if you're trying to run our games through virtual machine software.
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This makes it quite clear that GOG's service (which is as a distributor, not a developer) of selling games, applies to next-gen PCs and on modern operating systems.