It's not that sub defense is so great, it's that your time on the ground is very situation in Judo, and limited by time and the ref's discretion. Most Judo matches at all levels are a battle of executing a clean throw to win, with pins and submissions being secondary. Judo submissions consist primarily of armbars and gi chokes, and with gi chokes being impossible without the gi, most judo practitioners will be best at the armbar no gi - Ronda more so than others.
Sambo has a broader range of submissions that are legal, and there is more ground work allowed in competition. So while I am not as familiar with the norms of high level competition, I think you will see more submissions in high level competition.
You would be penalized if you just straight pull guard just to do it, not as part of a throw or submission attempt, and you would be stood back up. You also can't do it to stall or to avoid a throw. And it's not like you can pull guard and just stay there. If you go to your back and nothing is happening immediately, you go back to your feet.
By the way, I'm not an expert on Judo competition, but I've seen a fair amount and competed in one, and this is my understanding.