PHP did this (conditionally, at compile-time) with its ZTS builds, to allow multiple PHP runtimes in the same memory space. There is a not-insignificant performance loss from running this way, and so it's just not done, at least on Linux and similar scenarios where multiprocessing is feasible instead.

Although PHP7 does have some workarounds to improve the performance of this mode, involving implementing thread-local storage on certain operating systems..