Why is SRP considered to be resistant to MITM attack?
As has already been commented:

The fact that you can intercept and relay all messages does not actually constitute an attack on the protocol because you do not know the shared key afterwards. If the protocol partners encrypt their traffic with that key you cannot eavesdrop on it.

If this was an attack on the protocol none of the network hardware that is commonly used would be allowed to work because forwarding messages is exactly what it does.

Indeed, that means that participating in the protocol by relaying messages does not suffice for authentication.

Edit: It would be like thinking that all of your letters have been sent by the mail man. ;)