Yeah because the Thais don't start training in MT from like age 6 and they aren't any good at MT anyway
Not saying TKD/Karate aren't useful but there is no reason you can't start MT early. I've seen plenty of kids at MT fights going at it, I'm not keen on head shots as far as developing kids are concerned but there is no reason they can't be learning good technique and throwing to the body. Same with boxing.
Boxing and MT are easily good enough as a base. TKD is supplemental and not necessary at all. Jose Aldo to me is the best striker in MMA at the minute. And MT has spinning kicks and jumping kicks, christ they even have a handstand kick!
A simple and good reason for kids not to start MT, by the time the are in there 20's most Thai fighters that started as kids are heading to retirement. To much abuse to the growing body. The training is probably fine, but the fighting is were the problem start to develop from