They are reliable, easy to learn, low power, have an open source toolchain (and multiple open source programmers), a wide range of package and feature combos, T

They are reliable, easy to learn, low power, have an open source toolchain (and multiple open source programmers), a wide range of package and feature combos, TONS of examples and libraries (even without Arduino), require few external components... on and on and on. If they fit the needs of a project they are quite pleasant to work with and not every project needs a powerful MCU. I dunno if I'd pick them for a large scale product because of unit costs, but I use them all the time for hobby and even some limited production work-related stuff when parts costs are a rounding error.