security is expensive.
It is unfeasible to host the site in house, since it would require a huge capital upfront.
Plugging on and off an hard disk from a hot swap bay in a datacenter roughly takes 30 minutes.
A premium datacenter (rackspace) charge roughly 350$ per hour in sensitive datacenters.
It's much cheaper to do this remotely.
Nobody who isn't prepared to invest a huge amount of capital in security upfront should be trusted with other people's funds to begin with.