Why aren't those stories canon actually?
It depends entirely on what you accept as good enough. Because you have clear contradictions to well-established facts meant to be true in all universes (Such as the Beyonders vs The Living Tribunal) that are directly contracted, and also editorial comments that directly state Starlin's later works and solo stories have no place in the main Marvel Comics stories and are better seen as stand-alone.
Of course, you'll find comics that aren't denied from being in the "mainstream continuity" referencing them either directly or indirectly (Like Ewing saying a line from the Living Tribunal in his Ultimates run was meant to tie with Adam Warlock becoming it, only for editorial to say that can't be true and Adam isn't it in canon), this is just how comics works.
Marvel isn't univocal, you have dozens of writers, artists, editors and hundreds of other staff all working in parallel without perfect communication and doing what they like. So you commonly have these inconsistent moments that sometimes are retroactively explained or not, with somethings just being ignored because no one cares, and so on.
This is why I said that the cosmology split had nothing to do with canon, it was just a way of making sure that different views of the cosmology are represented in their own terms as best as some users can accept.
Because the fact of the work is that Marvel is a series full of contradictions that doesn't work with a consistent structure of the world across multiple sectors and for a long time. You can have someone like Al Ewing trying to map out the cosmology trying to pick up lines and concepts for over 50 years ago in the same era that a random writer just googled "what is the multiverse" and wrote a whole story based on a random article about dimensions that make no sense.
Marvel sells itself as a single world (Regardless if they are talking abouta multiverse), but the truth is that cosmological consistent isn't at all a matter they actually push as important enough besides some very broad points from a few runs, but that even then aren't fully respected in general.
But there are somethings even the editorial can't accept, which is how you have people like Tom Brevoort being of the constant opinion that Starlin's late Thanos stories aren't canon.
The question is, where lies the limit of the acceptable in this forum? The problem of a standardized Wiki is trying to figure out an answer to this and please a good enough amount of people with power to set-up the pages. Honestly, not much different from how some other things are set-up in the world.
Logically the actual answer is personal, maybe to you none of those contradictions matter and you can give an answer to how it all fits. At the same time, someone else might think differently due to having a different set of value. Normally it would just mean the two of you can stay away if you don't need to engage with each other, but are forced to met and discuss here for the sake of having an opinion written in a page in a Wiki that depends on convincing other people about your view (And the acceptance can vary even just by the week and those available to see it that time and the mood, it's not absolute fact).