Because there are also advantages. iOS gets away with forbidding JIT execution in third party programs, so they get away with exposing no APIs that allow memory segments to be executable unless they are signed, which is a great thing from a security point of view.

Also as there is only one browser engine, the system only contains the bugs that are in one browser engine. Most operating systems have bugs in the browser engine that comes with the OS, and bugs in the engine users actually use which is more bugs.