This is a multi answer answer:
The average 1000lb horse grazes for around 5-10 hours per day, and eats approx 4lbs of grass per hour. Assuming it's turned out 24/7 that adds up to about 20-40lbs of grass a day. (Lots of grass)
The time a horse spends grazing depends primarily on:
A) Type and availability of forage - if the forage is lower quality the horse has to spend more time eating to get the same amount of nutrients as they would from good quality forage
B) Level of nutrient demand - how much they need to eat to at least meet their nutrient needs.
C) Consumption behaviour - patterns of eating are developed in response to daylight/darkness cycles and other environmental cycles. (1)
Horses who are turned out 24/7 often eat on and off, while a horse who is turned out for an hour or two a day may be trying to eat as much grass as they can before they're brought inside.(1)
So basically, there are lots of factors that determine how invested a horse is in his meal. It may just look at you think "I could say hi but I don't know if they have food and there's food right here" continues to munch
Tl;dr If he's hella hungry or skeptical that you're a friend he'll ignore you.
Possible solution: have apples or carrots?
Myself (have a horse)