Horses are just like any other animal - they'll have different personalities and I am sure some do like to be ridden. Good horse owners do not keep their animals in stalls all day long just waiting to be ridden. That's cruel and will make such an active social creature go mental as they only sleep 4 hours a day. Instead stalls are used to keep them safe at night when their owners are sleeping. In the morning they can be turned out into the pasture. However very few pastures are big enough to encompass the territory a wild horse may wander. So being ridden will expose the horse to the exercise it might be needing, bonding time with its owner, perhaps even time with other horses if there are several horses being ridden, and it'll be a different environment to explore - all good and enjoyable things for an active brain.
You cannot compare a horse being ridden to slavery anymore than you can claim that working dogs are also slaves when they tend sheep, sniff out bombs and drugs, assist cops and airport personnel, complete agility courses, or pull sleds, etc. There are bad horse owners for sure, and has been for a very long time. It's in our language, "beating a dead horse" refers to bad owners driving their horses past the point of exhaustion and whipping them until they collapsed dead. If you want to cast a judging eye on these people then by all means do so - otherwise it's probably best not to critique owners who see their horses as beloved pets.