Because people tend to notice when one price goes up. They notice much less when there are multiple excuses and many prices are going up. It’s a lot easier to refuse to buy one thing than it is to refuse to buy everything.
We don’t have the kind of supply/demand price curve you read about in econ 101 because there are too many barriers to entry to starting any business. Once you’re established, you can either choose to race to the bottom so that both you and your competitors lose money OR you can implicitly agree to set your prices about the same as theirs. So choose, do you like more money or less money?
Yes, it’s partly inflation. And it’s partly the PPP. But largely it’s just greed hidden behind excuses with no real threat of PR fallout.