Literally because it goes backwards too.
Take the 40 ton scene, sure you could use it to get 10000000000000 ton MUI Goku. But you'd also get like 0.0000001kg Freeza.
And even if you ignore that, DBZ has way to many cases of lifting feats where they struggle to where when you work backwards, you get like 0.00001kg Cell or like 100000 ton dudes who cant even lift 10.
For example, Goku failing to catch the mountain Freeza yeeted at him. That is a cap for LS at that time, if you use multipliers backward you'd get ludicrously low LS prior to that point. This goes for later stuff too, where the backward multipliers contradict established caps. Or like, if we take an earlier feat, multiply it, you'd get like Class Z freeza saga Goku all while having a billion stated and shown anti-feats. And that applies to EVERY starting point, whether it's Goku moving a rock in DB, or lifting Piccolo, etc, it eventually spirals out of control and becomes hard contradicted both above AND below the starting point.
It's just not feasible imo, speed and power get away with it but LS guts itself.
But that's just my opinion.