Camille Herron is the only elite I've heard of who regularly does cooldowns after a marathon.
Meb cooled down after his second marathon, Chicago 2003, where he ran under control to get the Olympic A standard instead of racing all out. Most of the time, he can barely walk afterward.
For the top elites, one barrier to cooling down is logistics. The top finishers might have immediate TV interviews to do, and at places like Boston or NYC they're in the media center for a press conference within an hour of finishing.
But the main reason would be some combo of they're trashed (lots of them are already having trouble going up and down a few steps at the press conference) and they've just completed their goal race of the season and are likely going to take a break. Hobbling through a couple miles half an hour after finishing just isn't a priority at that point.