You can buy Bluetooth remotes for android that have this button - I have a few for just this reason.
I use a Fire TV 4K remote that has the Pause/Play and rewind and fast forward buttons. Works well for me. If you want to use the IR for power and volume, you need a Fire TV one time for the IR setup. But if you are using CEC or just don't need IR, you don't need a Fire TV at all. Just pair the Bluetooth and it works.
I not only like having a play/pause button, but also rew/ff buttons.
These dedicated buttons function differently than the up/down/left/right/ok buttons do on the "wheel" buttons that all remotes have.
The bluetooth remote I currently use with almost all android boxes is the remote that comes with the verizon stream tv gen 2. The play/pause & rew/ff buttons work on all devices I've tried it with, but some devices don't recognize speech through the microphone (although the voice button itself works).
Some firetv remotes have play/pause, but the firetv remote generally works in a glitchy fashion on other android devices.
The nokia streaming box 8000 comes with a remote that has play/pause & rew/ff buttons, but it is quite a big remote.
There are a lot of remotes you can get on Amazon with the future you want they are Bluetooth so make sure the toy you're trying to use the remote on is Bluetooth and then just pair the remote they're very cheap now
The AirTV Mini also has that button. As for why more don't, part of it is, of course, that Google doesn't require it, and doesn't put one on its reference remotes or on the Chromecast remote. Onn just uses Google's reference remote design.
Dont forget about Nexus Player and Nexus Remote ! 😂
Here's one with play pause button https://www.amazon.com/Rii-Multifunction-Portable-Wireless-Rechargeable/dp/B06XHF7DNQ/
AppleTV’s second-generation Siri Remote has a play/pause button too.
Are you looking for a remote or an Android TV Box with a remote that has a pause button. If the latter, I might suggest the TiVo Stream 4k. It has probably the best remote of any TV box. I have 4 different TVs, The TiVo remote works flawlessly on all of them, I almost never have to use the actual TVs remote for anything.
It's probably not part of the Android TV spec. So OEMs can have it or not have it. The original Shield remote didn't have a pause button either.
I think just to simplify buttons. It's not strictly necessary, although I also like having it, so it gets skipped to save space on the controller.