Finally, a good fucking post.
It's more due to the amount of repetition of those kicks. When you have kicked for that long, 95/100 times your kick is going to be pretty much mechanically perfect and every facet of balance or strength you need to deliver the power in that kick is well ingrained into you. Couple that with Muay Thai being your actual job as opposed to a hobby and that mindset of your life being Muay Thai, that helps someone develop a great deal more. The culture makes a big difference, the average karateka is a hobbyist, the average muay thai fighter is a professional, and yes there are more professional and ex professional fighters in Thailand than there are hobbyists.
I don't think the kicking technique of kyokushin and sanda for example are necessarily weaker, but I haven't really seen any guys with the same kicking power you get from Thai's around the same weight. I think that Muay Thai is REALLY only made up of leg kicks, body kicks and teeps (with head kicks existing but not being focused on) allows you for greater progress at getting very powerful in that specific area.
If I want to be competitive in taekwondo for example, I need a wide assortment of kicks because I need to score those points, so while I might be able to kick hard, it's not going to be the same as the Thai method of 'smash that body at all costs'.
It is absolutely a matter of training over natural selection. You won't find many Thais who don't have a hard kick. .
Since you can't view Muay Thai without rose coloured gasses, I'll name one elite guy for you then who does not kick particularly hard and that's Buakaw.
https://forums.sherdog.com/threads/...w-has-one-of-the-weakest-kicks-in-k-1.579695/