In the BBS reports it is also stated that
any Kingdom Hearts brought about by forcibly collecting/gathering hearts is
artificial and the one in KH 1 certainly fits that definition as well because Ansem SoD forcibly put it together by capturing hearts from destroyed worlds.
He essentially did the same thing Xemnas did, only the "type" of used heart differs.
As for goals and stuff,
Meta-wise the answer is clearly because at the time of KH 1's making there was no established lore and background for the concepts introduced there to be "fitting" in a larger overall frame, partly also because it wasn't until KH 2 that any at least rudimentary forward planning started to happen (Nomura was planning things for BBS while working on KH 2) but mainly because an actually overall frame
didn't exist yet.
Even if Nomura already had some rudimentary ideas in his head at that time, they weren't fleshed out in any form.
This is the reason why there are several elements/concepts present in the first KH which are presented as somewhat incompatible/disconnected with the rest of the series or not making much sense with later established lore.
In-universe this can somewhat be attributed to the whole memory-shenanigans of the Ansem/Xemnas split and both "halves" not fully understanding the main Overnort's overall plan and/or missing some of the finer details.
It is also worth to keep in mind that Xehanort (and many of his direct offshoots) are sort-of scientists who like to experiment and "see what happens" just for the kick of it and try out several possibilities to reach their ambitions.
Several things can thus be hand-waved of curious Norty McNort just making another experiment to see if it works or not. If it does, jackpot, he has KH and can fulfill his ambitions, if not, refer to the next back-up plan or come up with a new experiment and try again.
The end goal of gaining the power (and supposed knowledge/wisdom) of Kingdom Hearts itself to reshape the universe to something Xehanort and his offshoots deem as more suitable for their likes (aka essentially playing god) is something that applies to all of them as Ansem SoD, Xemnas and Master Xehanort all make reference to this.
How the individual variant of that main goal manifests is slightly different for each of them possibly due to the nature of the beings with Ansem SoD (as a Heartless) interpreting Darkness as the source of all power and the ultimate expression of it while Xemnas already has the whole "shape a new world"-stuff in general outline present:
Xemnas said:
All that and more. I'm carrying on what you yourself began, and I'm
creating a brand new world, one heart at a time.
These can be (as stated above) due to fractured knowledge from the split, possibly even a warped interpretation of the knowledge Xehanort may think he already possesses.
The latter gets also brought up in KH 2 when Ansem the Wise accuses Xehanort of being just as ignorant as he himself is about hearts and their workings and that the knowledge Nort prides himself of having isn't really worth that much in the end because of his warped views and interpretations.
So tl;dr, the main end goal is essentially the same of all versions, but how they individually go about it has some variances.