They tried that before, but they didn't count on Kirk's brain. Sometimes Spock's brain was under glass but that's another matter.
But then, the Susan Ross Foundation didn't count on George's brain, either.
Why wouldn’t they sterilize a planet before technology emerged?
They want to consume the life, but they need to find it. Their detection of space signals is far superior to ours, but they still can’t waste energy just hunting. A lot of animals are like that- they just sit still and wait for something to come by...