Silly, indeed. There are even pilots who have demanded Active Noise Reduction headsets and still fly around with an ear uncovered.
Pilots are more than a little fixed in their ways and those brought up on this method will not change. It probably originates from the time that intercom boxes only had a single volume control and there was a continual battle for all the crew members to get the radios plus interphone to a commonly accepted level. It was easier to give up on the I/P, just use the box for radio, and shout across the cockpit for intercom.
Even with the current, competent, audio boxes the Boeing R/T - I/P switch is a pain, out of sight under the yoke horn, and easy to misuse. The first time a youngster accidentally transmits cockpit conversation (probably about a hostie or the Ops Manager) on radio, the skipper will re-instate non-electronic conversation across the cockpit. The R/T - I/C switch found on UK and some European aircraft is a better gadget as it can usually be seen to be in position and it has a distinctive locking feel as it goes to the I/C position.
Flight safety would be served much better if the use of interphone was standard.