Anti-competitive pricing.
This shouldn't really be confusing this is first hour first day stuff if you study how and why monopolies exist and can persist.
Here's an instant example. Let's say I own every single gas station in your city, a thousand of them or so, and I charge 100% gross margins on gasoline. I do hundreds of millions of dollars in business.
You want to open a gas station with lower margins and undercut me. You pick a good location and open it.
Great, my location across the street can just sell below your cost unit you run out of money. Bye, see you later. Was that in your business plan? Where'd you get all that capital? What kind of return were they expecting, and how will you deliver it selling below cost for an indefinitely long period of time?
Also where are you sourcing your gas? Maybe I have already locked up all the current deliveries given my buying power with the refinery. Also what happens when you find out that nobody will service your pumps because I've locked them all up too, and am paying more than you can afford. And you can't afford anything because you're selling every gallon at a loss. Meanwhile the last couple years of 100% margins have given me a giant pile of cash I can use to do all of these things.
That's just a basic example. Sure yeah maybe you could somehow attack this market, with a bunch of capital, maybe there's like something you could do to compete with me.
But you're an ambitious and aggressive entrepreneur and that is going to be a TON of work. Why not just, like, start a different business?