Because if it ever needs to change, you're in for a world of hurt. Because useful stuff like that is worth sharing elsewhere. It starts with 2 lines, but then theres another useful function you'd like in another file. So you just copy and paste those two lines. But then you want that in a third file. Pretty soon you have this almost-library you're carrying around you've spread across a bunch of files, and now what started with two simple lines is now a mountain of tech debt.

Maybe you'll never write enough JavaScript to have additional utility functions. You'll probably never need to modify those two lines. But copying and pasting like that makes for quite the code smell. Because if you're copying and pasting that, the question that someone may never actually verbalize to you is what else in the code is copy and pasting instead of being turned into a shared function in a libray?