There's a few ways to "detect" VPN traffic, and you're missing some but port blocking is one of them. Rerouting over 443 is a possible workaround, but depending on the network architecture they can still detect VPN traffic using deep packet inspection.

Blocking ports is a very simple mechanism to prevent things and it doesn't take long for a business to grow into IT management that involves more sophisticated methods like DPI.

VPN protocols have distinguishable packet headers/metadata/handshakes/etc. DPI can easily identify and block those, or any other known protocols, if they have it configured to do so.