Because "gaming" headphones often aren't "just" headphones. They are headsets with a built-in boom mic

And a boom-mic is often way better than a crap wire mic or bluetooth headphone non boom mic.

There are still the trash ones, but there are also very good ones.

And a built-in mic is extremely useful on multiple situations : console gaming, tight space gaming (no place for a mic), or when there is only a single port on a device (tho a splitter, or hub could be used for a jack port or usb). There is also the convenience of just having a mic.

The issue now with all these headsets being "gaming", is because of the marketing.

Some headphones brands have put mics on theirs, to make them headsets :

Beyerdynamic with the mx300 : the tight clamp makes it a bit of a no go for me. The mic is as just between ok and great. The voice is full, but there is a lot of noise in there from the reviews I saw.

Audio technica : they have multiple of them. Latest one the ath-m50xsts. It looks like a circum aural headset, but is not. It's a on ear headset with ear isolation like a circum aural. Which is pretty bad for me. Tho the mic is the best I've ever heard on a headset.

They are both wired only.

And other brands not marketing as "gaming" headsets are either extremely expensive with strange mics, or have most of their production budget into audio and they pair the headphone with a trash mic to make it a headset.

In gaming headset brands, there are multiple ones providing software, mic, and wireless features enhancing the experience of the user. For example low latency high bandwidth wireless (proprietary, wifi-like 2.4ghz) connexions only exist in gaming branded headphones/sets. (high bandwidth = higher than bluetooth for the same latency).