Does this really happen?
Their desires could be far more cheaply satiated with a side project, but this way they don't have to use $boring_stack in their day job.
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.
These are all problems that you really don't want when your focus needs to be on the business. By all means adopt new tech where there is a real business need, and by all means avoid obsolete tech where you can, but be wary of the siren call of the cool tech stack when you're starting a new company.
On the flip side, stable and vetted popular frameworks also have security problems; and although these problems may be less likely, they are more bad people trying to break them, because when they do, the win is really big. E.g. Windows vs Linux viruses / trojans - I'm not sure if Linux is so much more secure to justify the difference.
Also new tech stacks typically evolve faster and are quicker to fix bugs. It is much faster to fix a problem in a young, less complex product, without having to deal with things like technical debt, backwards compatibility with millions of apps or endless discussions between committers about how it should be fixed.