Why not write in ABC

Simple.

I first learnt the basics of staff notation at the age of 11 so have been using it for over 50 years. I can see how a music looks and can get a sense of what it will be like. Staff notation gives a "picture" of the music, whereas when I see ABC it simply looks a jumble of letters to me.

I agree, ABC has its uses, certainly for transmitting tunes in a text only environment like this forum, but the first thing I do with an ABC of a tune is convert it to staff notation.

Mozart and Noteworthy don't work on anything but Windows, and are both one-person efforts, so they're useless as a long-term format.

True on the first count, and true to some extent on the second. The second point could be said of the majority of notation programs. However most notation software will read and export to MIDI which is a standard.