The reason people do not export a game to Linux or mac is because if you do not own that computer, sure you can still export it but you will not be able to playtest it, and then you will get a buggy mess. The reason people do not export to the web is that it can be quite challenging to get it working and it takes a lot of time. So to sum it up it doesn't just take two more button presses. Game dev takes a lot of time.