Those seas in particular? Because they are bounded by narrow straights.
A gulf has a more specific definition. A sea has a very broad one.
A gulf is mostly enveloped (engulfed) by land, but fairly open to its parent waters relative to its size on one side. Like a reverse peninsula.
A sea can be just about any large, salty, body of water that is between bits of land. They can have very finite bounds such as narrow straights and rivers mouths, more porous bounds like an archipelago, or broadly open to the ocean / other seas between islands or other land masses.
The question for me is “why is the Persian Gulf not a sea, but the Adriatic Sea is not a gulf?”
I guess part of it is that language changes over time, these things got named by different people over the millennia before they were well mapped. Some are translations from other languages, some are political decisions.
Podcasts, all running at 70% speed:
Bonus: I created shortcut that runs this playlist, sets the volume low, turns the speed down, and sets a sleep timer. I also have an automation that runs the shortcut whenever my phone connects to my Bluetooth speaker and my phone is in sleep focus.