It is not just training a horse, its trying to un-train them, but the training goes so deep. Also, OTTB sometimes get into this head space where they "flip out" under stress and go into race mode. Its very difficult to train out because it only happens in stressful situations, so he may be fine at home, or on trails, but you go eventing and he loses it.
Additionally, they are pretty "sensitive". Some feeds don't go well with them, it can be hard to put the weight on some of them, and they often need to be rugged a lot.
A lot of OTTBs have soundness problems, and they don't always show up until later. Their limbs have been put under a lot of stress. They also haven't been bred to be saddle horses, they are bred to run with a small amount of weight. They aren't made to put up with jumping, collection and carrying weight.
Finally, they don't sell for that much. In some places I guess they do, but a horse of another breed with equal training and potential will usually be worth a lot more.
If you are going to be putting the time and money into training its best to put it into the best sort of horse you can.
I know I am making generalisations but I don't think OTTBs are that suitable a riding horse. There are some lovely ones out there, I've seen them, but for each nice one there are 10 terrible ones, and you can never pick it until you've had the horse for a while, and tried to train it. To me, often their temperaments just aren't good enough.
From what I hear, over in the US you can pick up unbroken horses pretty cheap these days. I think you'd be better of with an unbroken Anglo, WB or something that an OTTB.
I liked Red Rok Spirit is the best there. I wouldn't really consider the others.
You can pick OTTBs up real cheap, often free. Look around and find one you really like if you want to get one.