Not being bound by the traditions of the large enterprise they used to work for, they're free to choose a stack that might help them be more productive. As an added benefit it helps recruitment - against the long hours and (relatively) low pay of startup work, they get to balance a mission and the opportunity to work with interesting technology.
My actual beliefs are somewhere in between. I think the 'fast moving' world of javascript frameworks is a major contributor here as almost anything you chose in the last 5 years is obsolete.
Not all tech stacks are predominantly web though. You might decide to build in Erlang/Elixir. You would be using something that's 30 years old, proven, not going out of fashion any time soon and the right tool for a lot of jobs. You'd be more productive in it than C++/Java, but that's what most enterprises would require.