It's because you would have no Bored Jumpers to play with otherwise. You would completely empty the ZS Mines of encounters for an extremely long period of time if you were allowed to dick and knock them all up on top of possibly getting pregnant yourself, and I think it's fair to assume there are as many people who want to dick NPCs exclusively just as much as they want to get bred into submissive euphoria. You could very easily just be wandering around and humping everything you come across and then want to come back later and... they're all pregnant, thus limiting the sex, and what you wanted to see.
It may sound ironic considering you can disable the Jumper encounters entirely by obtaining three vaginas, getting pregnant from three different buns in each one, then impregnating the fourth, but I'm taking an evincible leap in logic by considering very few people actually play with multiple vaginas. On top of that, players who will get multi vaginas and turn themselves into a walking Broodmother fetish will also have indirectly (and most likely) obtained mods and enhancements that vastly improve gestation speed, meaning you'll be pregnant for far less than the 6 months the Jumper would be.
When I write scenes I look at the scene I'm writing and try to consider the target audience of these exact scenarios and I think that as is it works perfectly. There is an argument to opening all four Jumpers up to being impregnated at any given time but I would need to write for that and if it was done it should honestly come with some unique scenes and one four-bun-orgy event, which I'm not keen on doing at this time.
