... I'm not asking about why you'd want one. - I'm asking about why you'd make a new one and who would pay for your work on a new one when others already exist?
But that is the same question. Not everyone is trying to maximize their wealth all the time.
Do you ask "why did Linus write Linux when there were other operating systems available at the time and no one was paying him for it?" or similarly the Atheos.cx guy? Gnome when there was KDE? Gimp when there was Photoshop? Facebook when there was MySpace?
The last one isn't comparable to the others. The original Facebook made by that other guy Aaron Greenspan arguably was just someone doing something because it was useful. Zuckerberg and co. made what is today's Facebook specifically to gain profit and power.
But your point is well-made otherwise. The answer to "why?" for uBlock and others is "because people care enough to do it."
