Because it is complicated. The game gets played to the referees whistle. That’s the way it’s been pretty much since association football came into existence. Getting into the area of chalking off goals scored is quite radical. And if we’re having a lad watching for whether it was a pen or not why not have a foul monitoring ref? A make sure the right team gets the throw in ref? And so on. How far should we really go back for the penalty in the scenario I outline? Because a foul or offside could be easily missed in the build up but if the guys watching the screen miss it too then what? Maybe the refs shoudl review highlights at half time and reverse lots of decisions prior to kick off of the second half.
The game absolutely should make use of technology, it just has to be realistic about what it can achieve by doing so.