Train 20 miles to go 26.2

This may be a silly question, but why do most marathon training plans (including Endorphins) only go up at 20 miles for marathon training? I’m doing my first full this November and before my first half this past March I ended up doing the 13.1 prior to the race which mentally made me feel a lot better lol. Would it be the end of the world if I did up to 23 miles prior to the full or should I hard stop at 20?

Edit to add: I’m a “slower” runner so I expect it to take me 5.5-6 hours to finish the marathon so I also considered doing a training run at 5hrs long (not worrying about distance) just to get used to being on my feet that long!