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But these Popes aren't universally accepted, now are they? Or do sedevacantists not count? As Fr. Cekada said, "what are we? Chopped liver?"
As for the Schism. It does apply insofar as it is the only other time in history where there was utter confusion about who the Pope was and who to follow. In this situation, many believe these claimants to be Pope, but the fact of their clearly manifest heresies have led to the question of legitimacy rather than identity. Hence why there are two more prominent camps on how to address this problem. It is the most confusing situation in Church history.
There's also the example of Pope Liberius, who was believed to have fallen into heresy. This led the faithful to elect Antipope Felix II, but this withdrawal from universal acceptance of Liberius did not delegitimize his claim to be Pope.

It's not just the SVs either.  Remember that UA means accepting the man as a rule of faith.  All Traditional Catholics have receded from these V2 papal claimants as being any kind of rules of faith ... theological disputes about ipso facto depositus vs. ab Ecclesia deponendus aside.

I keep pointing to cuм ex Apostolatus.  While one might argue that it's disciplinary, there's a latent principle there.  Holy Father says that a heretic pope cannot assumed papal authority even if he's accepted and given obedience BY ALL (aka universally accepted).  If UA is a thing, it would trump any disciplinary rules to the contrary, and cuм Ex would be moot if that were the case.

This notion of UA is constantly tossed out there as if it were some established dogma, but as far as I can see it's merely theological opinion.  I see no foundation for this in the Magisterium.

We have had historical situations (I think 2 of them), where there was a legitimate pope who went into exile (did not resign), and then another was elected in his place and was universally accepted.  But it was not possible for the second to be legitimate since a pope cannot be deposed.

And I hold that this "convalidation" principle is even more of a stretch, where you could have an illegitimate election, but then it makes no difference after he's accepted.  That would make all of those papal constitutions that indicate grounds on which a papal election would be null (e.g. collusion, simony, etc.) entirely meaningless.

See my previous post.  UA seems to be tossed out there all the time as if it were some given, dogmatically established.  I disagree.  It's a theological speculation that's fraught with difficulties, contradicted by the cuм ex, undermined by some historical precedents, etc.

And another aspect is ... universal by WHOM?  90%+ of Novus Ordites, by their own polls, have lost the faith, denying one dogma or another.  So, if the Arians, which some have suggested took about 90% of the Catholic Church down in its day, had managed to elect an Arian as pope, would that 90% acceptance count for anything?

So of those Catholics who have still the faith, a very significant percentage, even if they aren't SVs, entertain serious positive doubt about the legitimacy of the V2 papal claimants.  It is not necessary to be certain that they're illegitimate.  Rather, if you're NOT certain, with the certainty of faith, that they're legitimate, if you can't basically say that you're as sure that Bergoglio is pope as that there are Three Persons in One God, then you don't have the requisite certainty of faith to make their legitimacy dogmatic fact, which is what it must be for any dogmas these popes were to define to be certain with the certainty of faith ... and that's where this principle of Papa Dubius Papa Nullus ("a doubtful pope is no pope") come into play.