Well I can sort of see the thinking there - why would you spend money on a screen additional to the one you carry round with you anyway ? A lot of people are quite pleased to use their phones as the primary FPV display. Of course their DJI Go software doesn't work with all phones, but if you have an Apple one on the approved list, it does work most of the time, although few phone screens can throw out the sort of brightness you need to reliably see your aerial camera in daylight. Another reason is the portability factor - remotes with integral screens are often much bigger, meaning there is more to carry around. My TH is in a backpack roughly 3 times the size of the DJI mavic for example, which makes getting it on a plane quite difficult...
What is really terrible about DJI controllers is that they place the phone screen underneath the controls, which means your hands largely obscure it during use !
So the main reason I like my TH is the integrated 7" screen in the controller which means I don't need my phone, and have a very large, bright unobstructed display that works in all light conditions, comes with its own sun-shade, and has a custom Android tablet specifically designed to run the flight software. That works every time ! :)
CBR