Grab a script from the internet? Edit the shebang! Download some prebuilt software? Good luck finding /lib/ld-linux.so.2! Need to use some proprietary software you cannot edit? HALP!
Of course there must be some tooling to get around this, since stuff like steam is available (at least I hope), or else anything not packaged by nix would be a nightmare to install and use as a user. Any niche or domain specific software would become unusable. Take for example Quartus II or Vivado (just to name the two top FPGA EDAs), those two are not packaged by nix. Yes, there are a few recipes floating around, but those honestly scare me enough to not want to do it with more obscure tools like those for other, smaller brands, FPGAs.
(disclaimer: I never used nix, but that's my understanding from the article, and my own experience running proprietary stuff)