The move to QSG was not really that strange.
Guaranteed domestic trophies. Playing in a top 5 league. Very high wages. Paris is not a too bad city to live in when wealthy. France being close to Italy, culturally as well in many ways thus Buffon feeling (I imagine, even think that he spoke about this in some interviews) fairly content/safe/familiar in Paris. Close to Italy in case he wants to go back quickly. Climate wise there is not a huge difference between Paris and Torino (Northwestern Italy).
Good chance of winning the CL with Mbappé and Neymar around and a very strong squad on paper. A good and highly rated coach on paper in Tuchel. Only Areola (decent but nothing special) to compete with for a starting place. Can rest in plenty of games without any problem.
Today's result very few people expected. When Man Utd were up 1-2 the bookies (Bet365) had PSG to go throw at 1.10. Basically nothing.
There were no other options for him in Italy, no in Spain (elite clubs), no in Germany (only 2 elite clubs if not only 1 in Bayern and they have Neuer) nor many in England. He could have gone to China, UAE, Qatar, KSA, Japan, MLS but he wanted that last chance for a CL win (said so himself) even more so after what happened for him in the CL last season and the fashion he exited the tournament. Paycheck wise he probably earns as much as he could in those countries so not an incitement for him.
So not really so strange. Even less so knowing what a big competitor even a 41 year old Buffon is.
I much prefer him going to the likes of PSG (even though I don't like them) than him wasting him time in some joke domestic league in Qatar, MLS, China or elsewhere.