It's definitely a thing in some cultures (Italian I know for sure, possibly Mexican too?). I personally boiled down some lasagna meat for 4.5 hours just the other day.
Oh tomatoes? Well, tomatoes love to be grown in acidic soil, which increases the uptake of nickel and chromium. And they had better be organic, because common contaminants of fertilizer salts include arsenic and cadmium. But you don't want to eat organics if they're grown from waste biproducts of animal farming, such as commonly used bone meal, because those concentrate heavy metals.
A long simmer really brings out the flavor, my grandmother would simmer it for hours, at least half a day if not longer. And she used fresh tomatoes out of her garden.
Yeah, at the end of last summer, I had a whole pile of excess tomatoes from the garden that I wouldn't be able to eat before they went off, so I bunged them in a very large saucepan and simmered them until they took up 1/3 the volume, and put them in hot jars. It's a fairly common way to preserve them.
The word gravy is sometimes used to refer to other sauces. In (American?) Italian cuisine it specifically refers to a very slowly cooked tomato sauce that is traditionally made on Sunday. In Indian cuisine it refers to the sauce made from spices and vegetables. In Southern US cuisine it's more like a béchamel. The etymology of the word is unclear but there is a suspected link to old French words for spice and stew.
