The reason monitors have to be smaller, is because most people use desks against the wall.
If the desk is against the wall, then you can only sit so far away from it, that ultimately limits how large a monitor could be, before the sides fall outside your field of view.
This is the same as flat keyboards, the general population just doesn't know any better.
-- The correct layout should be the Desk in the MIDDLE of the room, with a 50-75 inch TV against the wall , used as a monitor
The small monitors are priced the same due to price stickiness more so than any technical reason.
I recently got 2 of those bestbuy cheapies $179.99 for 50 inch, 4k, VA-panels, very good blacks vs ips..
Using them as pc-monitors..
4K computer monitors and 4k tvs TODAY, have the same input lag numerics.
The feature missing from tvs vs gaming monitors, is ULMB and 120hz most of the time. It's missing due to the lack of demand, rather than technical limitation.