Wow this became bullshit really fast, the magic that is considered trash because it makes you heavier and it's literal second ability already does the opposite of that, you can't tell me NO ONE reached the second ability specially since the MC got it so fast
Who no one? Would they know even if some mofo out there was a master of gravity? In the first place, the general belief is that it's a trash skill that only makes you slower (heavier). Most of whom get it likely give up. If they continue as an adventurer, they might get eliminated by the dungeon early on, before they grow enough wits to just realise stuff on their own, by trial and error. Thanks to his isekai memories, the MC realised early on it's gravity, not slow yourself. So, he was open to new possibilities and applications from the get-go. You should keep in mind these are teenagers in an underdeveloped world. They are exceedingly ignorant. Even the arrogant merchant's daughter would be majorly stupid compared to a regular high school kid from our world. They are incapable of thinking outside the box, due to their limited view of the world and experiences.
Of course things are simplified/exaggerated for the sake of a narrative in fiction.