Looking for a video door intercom system for a single-family house with 2 monitors for 2 floors

If you want a video image, you need something to display it on. Of course, that can also be done via the phone. Or if the tablet is lying around somewhere. How practical that is, everyone can decide for themselves. It rings and you run through the house looking for the tablet. It wouldn't be for me. In that time, you can also just go straight to the door. Then you only need a button at the door and a chime inside. Also a solution, at the price of a case of beer.

No, why not simply just a display / a monitor in this format on the wall? A tablet has many more functions and would therefore be more expensive (camera, sensors, GPS, Wi-Fi, etc.).

It is not for this use. An intercom system is not a DVD player where you simply connect a screen.

Yeah, it probably really depends on the camera / door intercom system