There are no mountains around here, but there are still enough small hills to make me work harder. I do try to work those into my runs, but there are places I can run that are pretty flat. The parkrun I do has a bit of a climb so I want to be fit for that. We just have to work with what is available. I'm sure your legs are strong and running on the flat must seem easy.
!BEER
I've also thought about the effect of uphill running. I do think the muscles in both legs and thighs get strong by this, but I'm unsure if it's beneficial when running flat, as it's another technique. The best is probably the variation, and it's also great to escape from the asphalt sometimes :)
I expect there must be some benefit, but mixing it up is good. Running downhill can be hard on the joints. I do think different different types of leg activity work different muscles, e.g. running, walking, cycling, swimming. Just being active is a good thing.