Trump's camp has tried to make the age and competency of candidates a focal point, and they've also tried to highlight that the left is too left. What would happen if they got a younger candidate with a more centered running mate? Suddenly spin it all back and dump it on Trump and snag up all the undecided voters impacted by all the supreme court decision in the past couple years who would otherwise lean towards Trump
They'll say they're too young and too left. Or throw 5 other things at the wall and see what gets traction. Conservative attacks have never attempted to be consistent.
Politics isn't a left-right line where you win by being the closest to the center. A more center candidate than Biden, if such a thing even exists, would then further depress base turn out, which is already in a big flashing danger area. And the undecideds aren't centrists, seeking maximum center, they're some mixture of uninterested in politics and a grab bag of policy preferences that don't fit neatly into either side.