Intensive fast as possible farming afaik puts a lot of water in the plants so might be tomatoes themselves are more watery and have less flavour.

Not really sure but you probably shouldn’t be cooking down canned diced tomatoes anyway. They usually have calcium chloride added to them to prevent them from breaking down, they’re more intended to be drained and added to things you want intact chunks of tomato in.

I noticed the brand of crushed tomatoes I use is thinner.

Maybe they’re putting more water in somehow to sell you less tomatoes for the same money, like a form of shrinkflation?