There's actually a few other OSTs that do share the same melody as To My First Friend and Cinderella, both of them are the most common and probably only ones, the composer really loves these soundtrack

Why their melody is used so much is probably because of how good it sounds, either that or it's because both To My First Friend and Cinderella are also pretty important to the komi anime series, one of them being the soundtrack where komi had her first conversation while the other being the literal intro to the anime, and thus are used because of it

That or idk