And you dont know what security is.

"Let's just deposit 10.000BTC on an online site exposed to all sorts of hackers, and also take their word for it that they won't have access to it"

Basically this is your logic, its just a question of time until something will go wrong there, and it had, just look at the testimonials of people got hacked by using online wallets, many from this forum, so look around.