You mean, why not ask the AI to "find a way to use FTP", including either using a tool, or writing its own code? Besides the security issues?

One simple reason is "determinism". If you ask the AI to "just figure it out", it will do that in different ways and you won't have a reliable experience. The protocol provides AI a way to do this without guessing or working in different ways, because the server does all the work, deterministically.

But the second reason is, all the other reasons. There is a lot in the specification, that the AI literally cannot figure out, because it would require custom integration with every application and system. MCP is also a client/server distributed system, which "calling a tool" is not, so it does stuff that is impossible to do on your existing system, without setting up a whole other system... a system like MCP. And all this applies to both the clients, and the servers.

Here's another way to think of it. The AI is a psychopath in prison. You want the psycho to pick up your laundry. Do you hand the psycho the keys to your car? Or do you hand him a phone, where he can call someone who is in charge of your car? Now the psycho doesn't need to know how to drive a car, and he can't drive it off a bridge. All he can do is talk to your driver and tell him where to go. And your driver will definitely not drive off a bridge or stab anyone. And this works for planes, trains, boats, etc, just by adding a phone in between.