People can grow vegetables and simply eat. But bread is way too complicated.
There is a bakers’ dozen of big steps to go from wheat into bread. And multiple special structures needed too.
Same with beer. Wine makes total sense but how do you even invent ale? How are these common foods everyone knows and uses?
I was thinking “imagine if mediveal people knew how to boil seawater and sell salt” and now I spent 20 extra minutes in the shower.
You have fallen for the myth that salt was rare and expensive in ancient times. Medieval people did know how to make salt out of seawater. There were salt works all over the coasts of Britanny and Normandy during medieval times. Salt was not rare or expensive, except that they did need a lot of it because it was one of their prime preservation ingredients, so they needed barrels and barrels of the stuff, and that could drive prices up. But it was not because they didn’t know how to produce salt in enormous quantities.
Same goes for Roman times. The myth that salt was so rare and precious that it constituted part of the pay for a Roman soldier is wrong. It was because salt was such an important part of the diet and for preservation that it was given this way. They got grain and oil as well.
It’s also because salt is heavy as fuck, so transporting it from coasts and places with salt mines was expensive.