Yes it absolutely is impossible (even though it shouldn't be) so regulation is needed to make being a bag of dicks illegal or disadvantaged (like say the safety labeling on cigarettes, they're legal if they're labeled but no company would willingly label out that their product is dangerous).
Amazon can provide better prices and fuck over their sellers because they're being major douchebags in ways that shouldn't be legal and if it wasn't competing with them would be much more feasible because you wouldn't have to be just as much of a shitstain to compete.
But. Around the world people aren't really voting for regulations either, it's generally neck and neck between forces that wants far less regulation and more power to business and the ones that want more regulation and to reign in companies. So what I state here as obvious and needed is a hot topic for debate which blows my mind. We're callous fucking beings.
EDIT:
To answer your initial question, because it's the morally right thing to do to show solidarity with people that are effectively forced to work in shitty conditions in Amazon warehouses and to stop them killing smaller stores and retailers that provide far more jobs, spread wealth far more evenly and generally is vastly better for both the country and the world even though it might cost you 10-60% more on average and will be less convenient to you.
EDIT2 electric bugallo:
Also to clarify I'm not saying you're a bad person, you're absolutely not from what I can tell. You're just human like everyone. I'm not without fault and buy shit I really shouldn't as well, like stuff out of china when bespoke alternatives exist far closer. It's just painfully obvious with your story that we, the people, can't self regulate for the benefit of our fellow humans and the planet we all depend upon. Sorry for ranting towards you, and for any ill feelings I bestowed upon you.