Starting something new is quite rare, and if it's done, it needs to integrate into a whole bunch of existing stuff one way or another; whether it's builds, deployment, auth, database, payments, UI stack, whatever - large chunks of the design space have been explored before and are pretty much set in stone unless you have a lot of political capital to spike something new, or the company is dysfunctional and doesn't standardize on anything, so that it doesn't present consistently to its users and its developers are less swappable between projects.
In a startup, you usually don't have the time or resources to play with tech and learn cool new things. Instead, you're hastily producing another simplest thing that could possibly work, using predictable tools you already know in and out. You're lucky if it's RoR or Node; you could be stuck with Go or PHP.
In an early-stage startup you work long hours, and are preoccupied with two things: how to divinate what the users actually want (usually nebulous and contradictory), and how to quickly make it work this way. You can easily lose track of what's interesting in the broader landscape, instead of enjoying new hotness as it gets released.
If you want some leisure, join a large established company, work efficiently, and you'll have plenty of paid time to fiddle with the cool stuff developed inside it, and elsewhere. Or become so advanced that a large company will task you with solving something actually novel, not amenable to off-the-shelf solutions.
What the heck? My strong sense nowadays is that most junior-mid engineers would be excited to use Go and disappointed to touch Rails.