Because a large swath of the GOP, for certain the MAGA part of it, is dis-ingenous at best and rotten-to-the-core evil at worst and would rather lead a shitty, autocratic/fascist/dictatorial nation than partner or (gasp!) support the greatest democracy the world has ever seen. Threads like these are the little fuck-fuck games they play to achieve those goals.
Has Joe Biden lost his fastball? 100%. Are things going pretty well for US, all things considered? Absolutely. Whoever you want to say is leading the US at the moment seems to be doing a pretty fucking good job, again all things considered. But no, let's consider electing a brainworm-infected conspiracy theorist whose own family has publicly disavowed him politically (at least) or perhaps a 90+ times indicted, rape convicted, national security risk, potential Putin kompromat, diarrhea-of-the-mouth flim-flam man instead.
These SHOULD NOT BE LEGITIMATE OPTIONS in this day and age in this country. Period. End of story. Biden might not the best person we could get to be POTUS but he's damn sure better than these other two complete pieces of shit who are going to be on the ballot. One last thing. I can say with complete certainty that should Biden prevail this fall, he will voluntarily vacate the White House in 2029. I can't say the same for the other major party candidate. That should be the ONLY thing that matters right now.