The kind of person that lets a horse starve in front of their eyes doesn't usually want to pay for a vet visit and coggins test which is required to sell, or run the horse through a sale so that it can even make it TO a slaughter plant.
Years back when slaughter was really booming, a man starved a whole herd of horses right down the road from the Cavel plant. The "convenience" of slaughter plants won't stop irresponsible people from doing bad things to horses anymore than the "convenience" of your local animal shelter/pound stops people from starving and abusing dogs and cats. I'm hoping that people finally wake up a little bit and stop all of the excessive, and backyard breeding. This is the root of the problem. Slaughter simply makes it easier for these people to keep doing the wrong thing.
Huge remuda's are famous for culling a large amount of young quarter horses every year. They breed everything in sight, pick out the "cream of the crop" and dump the rest...many are sent straight to the feedlot, never even having a chance to find a home. What a waste. Broodmares are sent to slaughter, foals are born on the slaughter plant floor. GOOD horses go to slaughter. It's a shame.
As far as people stealing horses to take to slaughter, it happens. I know there are people who go around collecting free horses from ads in the paper to take to slaughter, it happened to a neighbor of mine. Nice old man said he wanted a couple of horses for his grandkid's...my neighbor gave him her old 20+ year old mare because the guy was so nice, and assured her that the horse would be well taken care of for the rest of her years, he said she could visit her whenever she wanted to. Well...come to find out, the horse was on the next truck to the Dallas Crowne plant in Texas along with a whole bunch of other horses that this guy got for free promising a "great forever home".
And...you are right, thousands of dogs are euthanized every day. If only horse slaughter was as humane...it's not. The transport alone is horrific (see link below if you dare). I know that other types of livestock go through horrible conditions, but most beef cattle, meat hogs etc. aren't named, trained, loved-on, and ridden...they aren't asked to give of themselves like a horse. Most horses learn to love and trust humans. When a horse is packed like a sardine in a trailer full of strange horses, traveling for hundreds, sometimes thousands of miles with no food or water to arrive at a slaughter plant (if they make it alive), herded up a chute...still trusting humans, only to have a captive bolt shot into their head (sometimes multiple times). Ultimate BETRAYAL.
Here are some GRAPHIC pictures of just what happens during transport
http://www.kaufmanzoning.net/foiaphotos.htmlDon't look if you have a weak stomach. It's horrible, but it's reality.