I set a quorum and if someone repeatedly misses then they get a direct “do you really have time for this game?” line of questioning.
That’s the way to do it. You gotta be willing to be tough with them
At least 3 players are enough for a session, and if you’re playing IRL, 2 can have a side adventure.
Online things are more flexible, but if folks have to get out of their homes, organize their actual physical books, snacks, and everything else, the game should happen unless it’s absolutely unmanageable.
I always make sure that my campaign contains 5 or 6 players, and I tell everyone we’re running if we get four regardless.
Sometimes when I’m tired or bummed out, I don’t really wanna travel to my friends’ for tabletop, but I always found if I did go, I’d have a good time and be happy I forced myself to go - knowing the session will happen anyway is a great way to encourage people not to flake out.