I guess up to a certain height or size you can pretend you have some personal space. I’m not a giant guy, but I long ago gave up any pretense that the area in front of me was of much use to me at all. It’s not enough space for me to lean forward and put my head on the table to try to sleep; it’s not even enough space for me to be able to lean forward and pick up something from the floor by my feet.
I’m 6’5” so the space is precious for me. I have been known to plant my knees as firmly as possible in the back of the seat in front of me and watch as the person becomes confused as their seat doesn’t lock in when they try to recline.