Because Apple showed up with a big bag of money, presumably.

Because those customers rarely have anywhere to go. because those customers would dump TSMC the instant there would be a better proposition?

Apple is the customer you want to maintain goodwill with.

Hell, there were times when Samsung was experiencing issues with sourcing their own chips for their own phones because they sold everything to Apple.

Launching new processes and building fabs for them is expensive and very risky. Pre-selling the output helps mitigate some of those concerns.

This is not the first time I've heard of TSMC reserving a very large portion of their production capacity for a single customer, especially in the early days of a new process node. I guess there's an inherent risk in being the launch customer for a new node (unexpected delays, low yield, bugs, etc.) that can only be offset by exceptionally large volume.

Maybe there's a gentleman's agreement among Apple, Samsung, AMD and nvidia that somebody's gotta buy those early wafers, but nobody can do that every single time?

It's business. They aren't going to lose goodwill for selling their product to the highest bidder unless they are reneging on contracts or something crazy like that.