Sideloading is just a normal term for a specific kind of installing, different than the default method of using the preinstalled store app. Practically nobody actually installs applications on phones from outside those stores. It's a useful and meaningful distinction, and much shorter than saying "installing apps by means other than official stores". You don't "sideload" an app on your PC because that distinction is useless, as it's the default approach anyway.
In fact, the term itself has existed since like the 90s, and has just been adopted for use with smartphones.
I didn’t say “nobody”, I said “practically nobody”, and that’s exactly the amount of users of those OSes/stores. Out of ≈7B smartphone users, even being giga-generous, maybe ≈1M use them (realistically, it's a fraction of that). That’s ≈0.014%, about the same ratio of people getting struck by lightning annually, or, you know, practically nobody.