The thing that allowed the Democrats to switch candidates was that the Democratic National Convention had not happened yet, which is when the candidate is officially locked in as the party’s candidate on ballots for president.
The Republican National Convention was mid July. Trump and Vance are locked in. To swap candidates now would be considerably more difficult for them (not to mention having to fight against a self centered toddler that will refuse to let anyone other than him run).
Swapping out candidates would be good for the Republicans. The process of swapping out candidates would not be.