Horses cost $$$, that's why. And don't like the actions of the minority reflect the majority. Owning a horse is a huge, h you g e, H you G E responsibility and commitment! Some horse and rider combos don't mesh, simple as that. Would you rather see a horse or ride become physically or emotionally hurt? I'd rather that horse be passed on to capable hand. Finances change! My dogs yearly upkeep, roughly $500. Horses eat up $500 a month for a SNACK, then some, especially if you have the accident prone type! Board by me is easily $700+. Now, lets say I come of hard times. Do you think I'm going to keep around a pet that costs me $5,000+ a year, no way. I'm not saying I'm going to dump them at the nearest open lot, I'd take as much time as I needed to sell to a responsible buy (to the best of my ability). Also, believe it or not, people DO out grow horses! If you want to move up to larger jumps and your horse can't handle it why not sell it to someone who is happy jumping that height?
All my horses will either be retired with me (if I have the room), sold/given to trust worthy people, or if need be, given the pink juice. Personally, I feel the need to "take care of" the horses who took care of me. Especially those that have special sentimental meaning.
What your wife should be outraged by is irresponsible owners and breeders. Not people who buy and sell responsibily. There are many people, referred to as "back yard breeders" or BYBs, that breed their pint-abianmares to their neighbors stallion quarter-walka-loosa because he has spots and they think their mare is cuteeeeeee. Well, the baby comes out and the owner swears up and down they would never ever get ride of it! Well, she spoiled the heck out of him, because she doesn't want him to hate her of course, and before you know it his is a monster. She posts a thread on horse form to help solve her problems because A) she doesn't want to discipline him B) She wants to do ALL the training herself and C) she doesn't have any money to hire someone to help her. Now here comes college, she wants to go away and she does. Well, her parents are not going to keep two expensive mouths to feed while she is gone for four years. Lets say they lease the mare to a nice family for the next for years. Now, the little stud (because you KNOW she didn't have time/money or felt bad for gelding him)... obviously no one wants him... he is a mix that doesn't really excel at anything, he's built pretty funny and worst of all he is a terrorist! Oh yeah, and a stud. Guess where that horse goes? Some sneaky "trainer" comes around and says he will train him up into something real nice! After all, he has been looking for a reliable trail horse. This horse just got fast tracked right to the auction house and might end up being rescued because some irresponsible breeder/trainer wanted to have something with spots and a piece of her mare!