This is the power gem, not the reality stone, in the comics the colors used to be different, and earlier in the story it is reasoned that Drax is too dumb to do more than subconsciously tap the power, so while inside him it should be more inert.

Although I just checked the wiki to make sure I'm not wrong on the internet, and came across this:

For some time, Drax tapped into the Power Gem to be the world's strongest green-skinned simple-minded powerhouse (during a period when the Hulk had much higher intelligence than usual).[15] The Gem usually remained in his stomach, but twice he was forced to regurgitate it from a blow to the gut, first from Gamora[16] and later from Thor.[17] After retrieving the Gem the second time, the Infinity Watch had the Gem installed in Drax's belt instead.[18]

jesus christ