Because there's nothing I love more than fiddling with obtuse settings menus just to have to do it again in couple of hours. This would maybe be acceptable if there was a charm/quick toggle like they do with Do Not Disturb, but I'd rather print it or write it than change the settings and then remember to change them back later.
I am surprised phones (I'll only speak for Android) don't make it easier to say "don't automatically sleep for the next 30 minutes."
I know individual apps can turn off sleep. The browsers ought to also be better at recognizing that you're looking at a recipe, and keep the phone from falling asleep.
My phones have a quick tile called "Caffeine" that lets you disable sleep for 5/10/30 minutes. I guess this might be limited to custom ROMs, but it absolutely exists.