The obvious reason is that it's neither practical, nor is it a good use of resources to enforce it.
It's not unreasonable to expect that chess.com will follow FIDE and USCF laws whenevery they are applicable and reasonably enforcable, but it is unreasonable to demand it when they are not.
Also, to expand on Cystem_Phailure's post #49, RetGuvvie98 was clear in his post to make the distinction that he was referring to those rules that govern behaviour, in apparent distinction to gameplay.
The members of the site have a reasonable expectation that the Laws of Chess will be followed. The burden is on the site to explain any deviations from the Laws of Chess that are not due to the nature of online chess.
I think that determining whether a draw offer is "reasonable" or not would require the involvement of an arbiteur, which is typically not feasible in online chess.
In your practice as an arbiteur, did you have a case when someone was actually penalized for making an unreasonable draw offer ?