People getting a financial gain such as interest over the coins they save there is the only reason. I can't think of anything else that people may find so interesting, that they throw away their full control over their coins. You said loans, but there are several other ways of how you can get a Bitcoin related loan. Think about certain Bitcoin lending sites, the community that you can ask for a loan, etc. No need for a service that is saying to be a "Bitcoin bank".
These lending sites are a disguise for scammers. The community where you can ask for a loan is not very much different from your circle of close friends where you can ask for a fiat loan as well. But that doesn't somehow make banks obsolete. A real Bitcoin bank will require you, as a borrower, to provide liquid collateral, and I don't think that they are going to consider altcoins as one such. In this way, they would protect the bitcoins they themselves might receive to deposits from Bitcoin users...
Personally, I think it is only a matter of time till real banks start offering Bitcoin related services (e.g. deposits and loans)
Yup the point of a bank's business is loaning money that you owned
Banks don't need deposits to give out loans. But that wouldn't work with Bitcoin