In the game, there is no default party, you have to create your own. What gives?

For some reason, the files are missing from the GOG release. I've heard they are included in the Steam version, but I can't confirm.

But it's a good way to try the game! That's why it's there!

I remember old versions of the GOG installer having the save with the default party, as well as the premade characters. Maybe they disappeared when cloud saving was implemented?

Interestingly enough, if you were to just add the first 6 characters in the roster, you get a party that's a good party for a Wizardry 7 veteran to try. That party has many of the classes that aren't that good in Wizardry 7 (Fighter, Psionic, and Bishop, for example), and it even has a Gadgeteer, so it would be a good party to use to see just how they made those classes actually useful this time around.

It's way more fun to create your own party, instead of what the developers thought your party should be. That's just my two cents.
I have created an ultra-powerful MDP (Magic Damage Party), at level 35.

Pregens are great, both as a way to get quickly into a game before theory-crafting and as a way to improve skills. Since pregens are often unoptimized, they can be used to make a game more fun and challenging. Of course, often is a key word there, because some games make pregens more powerful than what the user is allowed to create. Character creation is often the appeal of many an RPG, of course.

there is a pregen party in the gog build but the location moved after Windows 7 so the install fails because Microsoft moved where documents live in W8 and above

Gog is well aware but hasn't bothered to fix it

Curiously enough, the Mac version has a default party.