That's an interesting question, "why write new music." In a sense that could be made more basic by asking "why write music at all."

I was at a local community-band concert last night (https://firststatesymphonicband.com/). One of the songs that the band played was written by one of their own members. He received no royalties from it. He made no money from it. He just enjoys writing music and the band - all volunteers - simply enjoy playing music. The answer to the question "why write new music?" may be as simple as "because they enjoy it and it keeps their brain alive."