The most common excuse is "So they can grow up together." (Honestly, I'm still thinking about Dancer, as they got brought up).
People do buy foals for their kids - and it almost always ends up in a disaster. No matter how good a young horse's temperament is, if the rider does not know what they are doing, they will ruin the horse. We have a saying: Green plus green equals black and blue. Meaning that a green rider on a green horse is likely to end up on the ground.
Horses that are used to teach beginners to ride are carefully selected, older animals who really know what they are doing and have a patient temperament so they can handle riders who do not know what they are doing.
The excuse Martin used was so the horse would be trained to carry a disabled rider from the start.
Sorry, George, but it does not work that way.