Many, many places don't have that competition. Plenty of communities are at risk of losing their two local grocery stores and seeing them become just one.
Grocers are not evenly distributed, so while you are correct that at a national level there exists competition, it doesn't exist everywhere.
And while a monoculture in an area might represent an opportunity for a grocery store to disrupt things, the people living in that monoculture gain no benefit from that opportunity - they have to live with the monoculture until someone maybe fixes the problem.
Grocers are not evenly distributed, so while you are correct that at a national level there exists competition, it doesn't exist everywhere.
And while a monoculture in an area might represent an opportunity for a grocery store to disrupt things, the people living in that monoculture gain no benefit from that opportunity - they have to live with the monoculture until someone maybe fixes the problem.