An operating system should never overallocate memory because one cannot build reliable applications and infrastructure on top of a kernel which is lying to the application.
You cannot make reliable systems thinking like that. The point of resilience is __not__ to rely on correctness of kernel's advertised behavior nor correctness of your assumptions about it.
One cannot build highly available systems and networks on unreliable, lying software. Basic guarantees are required. If one has 10,000 systems and one loses power on even a portion of such lying systems without basic guarantees, no amount of distribution will guarantee data consistency. I'm no stranger to building very large distributed clusters, but those builds start with an operating system and software which do not overcommit and which are paranoid about data integrity and correctness of operation. In fact, I'm specialized in designing such networks and systems, from hardware to storage all the way up to application software.
Many databases rely on the overcommit being possible.
For the general case though, because of the way programs have been written, it is easier to have overcommit on.