Seems like it depends on people maybe. Some are very clumsy/unlucky, others not so much.
I have a cousin who's dropped some of her phones in the toilet. Twice. I understand that some people use their phone in the toilet. I don't understand what position you have to be in so that you could drop it in there.
As for myself, I have owned mobile phones for more than 15 years now, only one of them suffered significant damage, and that would not have been fixed by a case: At some point, the thing just stopped charging for no reason (except that it was a $10 phone, which probably was the reason). It is possible to be careful. If you have animals/small children (I have 3 of the latter), a bit of habit says your phone should generally be in a place where they cannot easily reach it: your hand, your pocket, on top of a shelf in a closed room, etc...
Additionally, most of my phones so far are in the price range of the "typical" iPhone case. I would not buy a $30 case to protect a $80 phone, that would be stupid. I guess it's different if you own a $1000 phone.