Because the cost barrier is generally much lower on a console than a laptop and many people would rather play on their TV over laptop/PC. Also, being able to control a standard set of hardware is beneficial for developers.
You could still plug it into a TV (like the Switch), and in laptop mode, not only would developers have standard processing hardware, they would also be dealing with a standard resolution and refresh rate of the screen. That'd be a nice juicy treat for development.
As for cost, yes you're right! That's a problem.