I think only google pixel 8s have this enabled. What phone do you have?
Discussion
7pro
That's the issue this feature is only enabled on the pixel 8 series from what I read 7 pro has the hardware but google blocked USB displayport in the source code.
Maybe with release of Android 15 it will be enabled but as far as I know now only pixel 8 phones can use this feature.
It kind if blows my mind how slow google has been with desktop mode. Samsung has had samsung Dex for years now and I think its a killer feature.