Some "flagship" and higher-end-midtier phones cheap out on the USB connection. USB 2 over USB-C with USB-PD for fast charging. No video out, slow data transfers.
Maybe when desktop mode becomes more common there will be an incentive to fix the shitty USB situation.
Cheap phones probably won't really have the power to effectively multi-task so I imagine cheap models would rather disable the feature than leave the user with a bad UI.
> USB 2 over USB-C with USB-PD for fast charging.
Sometimes you're lucky to even have conformant USB-PD. For example, OnePlus for a while had "Warp charging" and the phones wouldn't accept high power over regular USB-C PD.