Apple continued to follow suit in the laptop space with the introduction of the MacBook Pros with Retina displays which made it the first full-blown computers with these types of screen. However their desktop computers are not yet available with Retina displays, and some manufacturing and cost challenges are probably the main reasons why this hasn’t happened yet
If Apple made the screen bigger it would have either added pixels – forcing software vendors to write and support new versions – or it would have lowered the resolution from Apple’s famed “retina display” specifications