You can thank the Great Depression for the thick, fake stuff Americans love to pour on mashed potatoes. Pre-made gravy was part of the effort "to simulate meat in the diet of starving, depressed, out-of-work Americans":
After a series of failures with beans (though one of these lines of inquiry did lead, decades later, to the relatively well-received tofurkey), and some predictably mixed results with whey protein, Gravy Master was introduced in 1935. Caramel color, sugar, spices — with some flour and a pat of butter to make a roux, it would make a thick dark gravy. … When they added hydrolyzed yeast extract (like autolyzed yeast — the main functional ingredient in marmite) a few years later, the deal was sealed. Hydrolyzing yeast tears open the cell walls and results in free glutamates, essentially MSG, the umami flavor that makes your brain think it’s eating meat.