Adding RAM or changing hard drives/SSD (to bigger ones) is not generally harder on laptops than on desktops, while CPUs and GPUs are not generally even meant to be replaced on laptops, or definitely not upgraded. I recall there are some such gaming laptops where such options exists, but for 99% of laptops that is not the case.

However, when I think about my past desktop PCs, by the time I decided it is time to upgrade some vital component in it (CPU or GPU), usually at that point it made more sense to buy a whole new PC anyway. Quite often the slot technology etc. had already changed so a new bleeding edge graphics card wouldn't necessarily even fit to my old PC because of having wrong kinds of slots, or new CPUs wouldn't fit into the CPU socket that my motherboard had. Or even if they did, they would still not work optimally as e.g. my motherboard was missing some vital new feature that newer motherboards had that supported those newer CPUs/GPUs or new memory types etc.

Heck, even once when my PSU died and I bought a new one, I realized only at home that the power connector standard (to the motherboard) had apparently changed in the meantime, so I needed to hunt down for some kind of adapter that I could use that new PSU with my PC. Luckily I did find such adapter so that I could bring my PC back to life with a new PSU.

Then again, since I haven't bought new desktops for a long time, I don't know if the slot/socket/memory technology changes that often anymore, as it did back then. But yeah, all in all, desktops do offer more power per buck and easier maintenance, while laptops are more compact solutions and offer the portability (which are quite important to me, I love it how easy it is for me to move my gaming laptop, even just within my home to a different room, or even take somewhere if needed; or just close the lid of my laptop and push it to the cupboard if some friends come over).

Even more important to me is that someone else guarantees that the whole system, with all the different components, works together without a hitch. If I buy different components from different vendors and then there is some freak incompatibility problem that this GPU doesn't go well with this CPU or motherboard, then I don't have to try to find out what is the culprit. If I e.g. complain to the motherboard and GPU vendors that their components don't go well together, they will each just say it is the other vendor's problem.

When some PC store assembles the system for me, then they have to figure out how to sort out such problems, or alternatively contact me to negotiate whether to use some other component instead which doesn't have compatibility issues.

Doesn't Germany have PC stores that assemble (and test) the PC for you, from components that you have selected from their inventory? Here they usually have some default options (one for basic use, a more expensive one for gaming etc.) and either you go with one of those predefined setups, or then ask to replace some of the components in those default setups if you want some other GPU or different kind of or more RAM or HDD instead of SSD (or vice versa) etc.

So the only options here are not either build a PC from components yourself, or buy a generic HP desktop with no options. There's the third option between those two extremes. (Then again nowadays there seem to be less such stores than 10 years ago, but they are still there...).