Having a single flat price is/was due to labor prices being high enough in the developed world to ignore potential profits from price discrimination.

When my grandparents went to the market in the developing country they immigrated from, they would bargain for everything, and every customer got a different price.

The developed world was rich enough that grocery stores didn’t need to waste time doing this, and could simply price high enough to earn a consistent profit margin and expect consistent sales. They did engage in price discrimination via coupons. Just not individually, until smartphones and apps came along.

Now that automation can handle a lot of the price discrimination, expect more of it, everywhere.