It actually really is though. Counterargument time.
From the jungler viewpoint, the lane that is winning is the easiest lane to gank, given the inherent laner advantage and more-than-likely allied vision control of the area alongside lack of enemy vision.
The bot lane, especially when pushed to tower, is by far the hardest lane to gank, given that you have 2 laners each with their own trinket wards and later a person literally dedicated to warding. The bot with the advantage will have ward coverage while yours won't, meaning that both you will be spotted coming in and you won't know if the enemy jungler is around or not cuz you have no ward coverage bot. Not to mention that usually the winning bot lane will be winning poke trades making ganking for your bot lane even harder since they'll have less starting health for the gank. Most of the time ganking this lane without flash is damn near impossible.
This holds true for top lane as well. Top lane matchups are usually heavily skewed the moment either side takes a lead. This means that ganking for a winning top lane is extremely easy while ganking for a losing one is extremely difficult.
To top things off you don't stand to gain all that much from shutting down a kill lead vs what you stand to lose. If it goes well despite the disadvantage, you are only putting that lane at a net neutral state. If it goes poorly, you just made the lane unwinnable. Alternatively, if you gank a winning lane, it becomes unwinnable for the enemy. It's easier, far less risky and doesn't end in the possibility of making the lane unwinnable if it somehow goes poorly.
All-in-all, there's a reason most junglers typically stay away from losing lanes--there's just too much risk with too little potential gain while simply ganking a winning lane is basically risk-free and gets your team some risk-free extra lead somewhere.
I don't believe there is anyone who doesn't want to help that losing lane reach net neutral or even convert them to winning, but doing so is very risky. This is why junglers just gank winning lanes almost exclusively. Noone cares if you continuously get kills on a winning lane, but the moment you fail a gank on a losing lane you're setting yourself up for endless unreasonable flame.