Cos the other parts would still be working. They're not sure which part controls things like sociopathy or sexual predation.
I saw this program on people who've had lobotomies (they used to do LOTS of them apparently for flimsy reasons back in the 1950s) and one guy had a past with violence and drugs afterward. The lobotomy had made him bitter, he said. It didn't mention rape at all, and by the taping he had turned his life around. But it did seem like it didn't cure violence.
His stepmother got him the lobotomy as an early teen, for things that wouldn't even get a teen an appearance on Montel today. She snuck him in there. His dad was against it. I mean I'm not an expert and can't say for sure but there's enough of a risk that I wouldn't say to rely on it.