Mini-bosses in the overworld can afford to be far more lethal/challenging because all of that content is essentially 'optional' and you can avoid or engage any time you wish. And ideally Ganon feels challenging to them, without feeling overwhelming. Others may have tried to finish the game well before this point, collecting fewer hearts, finding fewer fairies (so significantly worse armor), collected fewer powerful weapons because they had difficulty beating Lynels, Hinoxes etc. Some people could have 84 armor, 27 heart pieces, all the best weapons, complete mastery of the combat system and just breeze through it. There is probably some expectation that the player will have completed the four divine beasts before attempting Ganon (I suspect only speedrunners would naturally play the game without doing those despite them actually being optional), but they really have no clue on how many armor upgrades or heart pieces you may have gotten. I actually think it's kind of an intentional design decision for the 'open-air' style they were going for.
