Why isn't score notation used much in music software nowadays?

For traditional instruments, the standard 5-line stave is generally sufficient. However, when incorporating multiple instruments in a audio workspace that may require different types of staves, attempting to display all the notes can result in a chaotic and confusing visual representation. To avoid this issue and ensure better readability, it is more practical to simulate the notes like MIDI notes, which can effectively accommodate various instruments without overwhelming the user.

It's probably just more accessible for most people to use a piano roll, since they don't have to read music.

Personally, I've never found a music notation program that was as useful for composition as a pencil and staff paper. Obv after writing you're going to want to use music notation software for presentation.

GarageBand supports entering notes. Try it and see how much simpler the piano roll is