Why? What we want is all the remaining cars going full pelt into corner one. Much more fun. We don't have to sanitise everything!
Because it's not fair

and yes, I'm still bitter and angry about Abu Dhabi 2021! I understand perfectly why they did it for TV, but in this case, Masi should have told both Lewis and Max that they were going to be allowed to race each other in a one lap shootout and that they had to be in 1-2 position for the restart, and that they were both free to change tyres.
But I've always been a supporter of not allowing cars to change anything during yellow or red flags, or if the safety car is out. If you get stuck behind lapped cars when the SC is deployed, sorry, that's just racing life, they shouldn't be allowed to unlap themselves.
The argument for allowing tyre changes has always been that it’s a safety issue. If cars have potentially run over debris which could have caused punctures, you’re risking more crashes by forcing them to continue on their existing tyres.
I'd suggest that in this case, you can calculate the time needed to pit normally and apply it as a kind of stop-go penalty. It's hard to explain what I have in mind, but you could say "okay, 2.5 seconds to change tyres, 25 seconds lost in the pit lane, so you'll incur a 27.5 second penalty if you change tyres, to be applied at the end of the race or at a following pit stop".
In essence, there wouldn't be a free tyre change, and hitting debris would be a racing incident. Even if there's a red flag and then the heavens open up, the decision would be down to the teams: do they change to inters or wets, or do they leave the original tyres on?