Whenever i open my Bluetooth headset mic in any sort of android game. The headset stops working. Why does this happen?

Bluetooth has rather limited capabilities for audio.

For things like phone headset connections, It works either:

In A2DP mode (Advanced Audio Distribution Profile), which has one-way stereo at reasonable quality; it's one-way so no mic capability.

Or, 

in HPF mode (Hands-Free Profile) which can carry two-way mono audio.

That mode is usually tied in to the telephone call audio system in a phone, which is a separate thing to normal audio for games or music.

When you enable the mic, the phone is likely switching to voice command or telephone call mode...

Some things other than phone calls may work with it, but it depends on the phone operating system and it's significantly lower quality than A2DP.