They would— it would allow them to accept more low fee transactions, thus increasing their fee income in that single block, by making the space non-scarce— though in the long run it will make miners less, at the time of forming a block it would make them more. There would be no rational reason to not gobble up the fees as as fast as you can unless there was cartel behavior to force lower sizes anyways. Maybe the cartel regulating miners would choose better limits, but it would mean that we'd have to trust the cartel to behave wisely rather than a set in stone rule which produces a market for fees by fixing the maximum size.

That is all in regard to why it should work the way it does— the reason it does work the way it does is just a result of the principle of the system. Miners are only depended on for ordering.