A) they believe in the idea
and / or
B) how much money there is to be made having people wear them.
Smart wearables as a general category of hardware have an awful rate of success, and hardware is much more expensive to get into than software. So, there's got to be a lot of money in the data consumers will be producing.
That's the part that scares me much more so than the random perverts using them in public for unsavory candid photos.