That's because it is (more than) pretty stupid to let users install
software at all. The job of system administrators is to "manage" the
systems they are responsible for. With Windows systems that requires
that "ordinary users" (i.e. everyone whose job is not officially
"system administrator") _MUST NOT_ be allowed to install new software.
Sadly, extraordinarily few Windows system admins actually have enough
nouse to realize this, and most of the few who do cannot get enough
management muscle to back such a "draconian" policy.