Before I got my mare she was on pasture 24/7 - she was MORBIDLY obese, with no grain or hay supplemented, she had large fat rolls and dangerously close to founder. She is seriously allergic to bugs, when I got her she had rubbed her face, neck, mane and belly completely bald and scabbed, raw and oozing.
If I were to force her to stay outside all day every day she would be miserable. After a few hours outside foraging around and playing a bit, she's at her gate nickering and pawing, desperately ready to come in. I ride her and exercise her, and soak her in fly spray and keep a fly sheet on her. But she is not happy outside. Putting a horse like this on pasture would be a death sentence.
My Belgian has a docked tail and can't keep the bugs off him. After several hours in his field he's very ready to come in. I'm not going to ignore my horse who's desperately trying to get my attention just because I have some ideal in my mind that horses should live outside.
This blanket statement that horses need to live on pasture is silly. I have 3 horses who would beg to differ.
My third horse has the indoor/outdoor stall, I leave his door open all day and let him choose. When I get home, all his waste is in his stall and so is he. He doesn't go out, despite the grass.
My horses get free choice hay (except my fat mare, she's on a strict diet) and the feed they need and of course free choice water. They are exercised regularly and groomed and cared for. They have their feet done (all have exceptional feet). They have the vet requirements met. They are good, happy horses. Pasture is not the end all be all for horses- in fact for some it's detrimental.