Usefulness of "dangerous" moves
why haven't we seen more people landing badly (or purposefully) on legs and breaking them?
Because it's actually really hard to cause damage with these moves, and a lot of that is just luck. They're banned because a small but significant percentage of attempts will, due to a the opponent's split-section reaction but also chance, cause serious injury. But it's not like they're "overpowered" moves in a videogame. The majority of scissor takedowns result in no injury. Checking a kick results in a broken leg maybe one time out of a thousand.
Another aspect is that both of your examples, the scissor takedown and jumping guard (which is, I note, only banned at white belt), have major drawbacks in MMA. The scissor takedown is quite hard to set up from striking range, and if it fails you've pulled a bad guard. Both pulling and jumping guard put you underneath someone who would love to hold you down and hit you until the end of the round while you squirm to get back to your feet – tactically, it is a choice riddled with problems. Jumping guard is worse, because it gives your opponent the opportunity to slam you. Few MMA fighters want to take the risk to themselves that these techniques present.
Different risk/reward profiles
Any sport that involves getting punched full force in the face is, by nature, going to attract people more willing to be injured. Grappling sports are intended to be easier to take part in without serious or permanent injury. Therefore more techniques are banned in grappling.
Striking sports like boxing, kickboxing, and MMA involve more infrequent competition and much more acceptance of risk than grappling. (This is somewhat less true in a Thai context, and lower-contact striking sports like tae kwon do are somewhat of an exception, but overall this is overwhelmingly true in the West.) The sport is designed for participants who want to trade safety for freedom and realism.
Expected competition participation
Judo rules are intended to be universal, used almost identically from community tournaments all the way through to the Olympics. The only difference between a white belt adult competing in my region and the world championships was that armbars weren't allowed until sankyu. Global consistency is highly valued.
Another core value of judo is safety for the massive volume of participants. Judo is almost as popular across the world as soccer, and it's expected that judoka will compete often and at all ages. The rules have to take this into consideration. Therefore jointlocks are limited to the absolute safest--those attacking the elbow, which come on the slowest and do the least long-term damage--to minimize the number of life-altering injuries caused by the sport. Similarly, many throws like kawazu gake which endanger joints are banned, because the large number of worldwide judo competitors would cause a large number of risky throws to be attempted, and thus a non-negligible number of long-term crippling injuries. Allowing the technique would make judo much more of a niche sport.
At a guess, MMA has 1/1000th the number of participating competitors per year, maybe fewer. The number of people put at risk is almost not comparable.
