It's called Capitalism. If I can make a thing cheap and sell it for a high price and still undercut everyone in the same market I will, and do. My average price is usually 5k gil. I make millions with little effort, and no need for bots, I just gather my own materials.
Here again, we have a demonstration of people not understanding Opportunity Cost; time is the ultimate cost even if you gather everything yourself. The time that you spent gathering stuff yourself could have been used to do something more profitable.

To the OP, diversify the products you sell and try to sell across the DC. Undercutting doesn't matter as much when you sell diverse products across the DC.