Because it hurts them more, teams (especially contending ones) will try to do everything before a buy out. Can you provide with some examples where a team gave up assets for a player, and then bought him out the very next year when the player was making something close to 2.75x2?
Like I said, they'd retain him further and flip him if it gets to that point, teams don't want to have dead cap space for no reason. There is no risk from the Habs here. I'm also not following your logic in your doomsday hypothetical scenario.
If the Habs buy out Anderson next year, they get stuck with 1.9/3.4/1.4/1.4 dead cap space for the next 4 years. If they retain 50% and trade him to another team and this other team buys him out next year, Habs will now be stuck with 0.95/1.7/0.7/0.7 dead cap space for the next 4 years instead.
There is zero risk for the Habs, it's only upside. Either they only have to retain 2.75x2, or they have a dead cap hit of 0.95/1.7/0.7/0.7 which is objectively better than buying him out on their own. No Anderson, money saved no matter the situation, and gained assets all for the cost of a retention slot when we have 2 more next year.