Good question! Theres lots of reasons!
1 - I genuinely feel happy when I see something we created help other people to create.
2 - I find it super challenging, constantly trying to be render things faster, whilst hiding away complexity from users (not everyone wants /needs to know webGL). can be frustrating, but ultimately makes you feel warm and fuzzy inside when you figure out a new way to do something thats faster and easy to use.
3 - I get a serious education in how the team manage communities, and how to work with other devs.
4 - Team is cool smart bunch o'people. I like interacting with them
5 - Its helps our agency from a business perspective through increased exposure.
6 - Its ultimately helped me become a better dev.
7 - We would have build something like pixi internally anyway as it helps us make what we need more effectively, we just went a step further and shared the code
No plans on stopping anytime soon!