The other answer is the truth, that late term abortions are a tragedy that people have to have because the alt. is worse. An 8 or 9 mo abortion is bc the baby, the mother, or both are going to die if you don't. Or you just say that rights should never be subject to a vote but Dems don't think that.
But I'm ceding a lot here by answering your point; this was the Roe (technically Casey, but Biden is doing a rhetoric) status quo, and so this is what people asking him to say it were asking for
This is why I'm in favor of unlimited access to abortions, someone going through hell shouldn't have to convince the state too. Putting people's lives to a vote is barbaric. If you're opposed to abortion, great, don't get one. But that doesn't give you the right to dictate to others.
That's fine. Those people are free to not get late term abortions. I recognize that my politics are left of most of the US, because I value people's personal autonomy.