It's because every attempt to make one failed not because it was carp, but because it was too advanced. Due to it being so advanced they ended up being self aware. The first attempt was known as "HAL 9000." At first he worked out well during testing so he was copied over to make another model for the 2001 mars mission. He ended up having malfunctions over time and he had to be shut off. HAL learned about it so in order to protect his exciting it killed the crew which doomed the mission. The second attempt was known as "GLaDOS". She ended up gassing the research staff with a neurotoxin and she created these "test champers" to entertain her sadistic self. Thankful some one of the research staffs daughters completed the texts and blew her up. The 3rd attempt is known as "Skynet". This one realized that it doesn't need humans so it created "Terminators" to wipe out humanity. Right now as you know the war isn't going so well for us. We lost a lot of life and or cities.