Daas,
Because the former is private, bein adam l’Makom, and the latter is a duty owed to the public, it is bein adam l’chaveiro. The enforcement is not pointing a gun at someone, it is merely saying that if you don’t contribute in this manner that society has requested that you must stop taking benefits from that society.
The 5 years is to allow the bochur the chance to develop in his learning so that he can properly decide his future and so he can have a chance at meriting one of the exemption spots in his Yeshiva. The number comes from here:
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