This answer is largely copied from my answer to another related question:
To improve the community I'm a part of.
The direct incentive is clear: to help others with their questions, and be helped with mine. (It's what brought me to RPG.SE in the first place after all.) It's pretty much the same incentive you have on any Q&A website. To the extent that the site continues to enable that, you can use it for that purpose. But your underlying question is more about why you should do that here instead of on another site.
Personally, as a moderator of RPG.SE, I do what I do to make my part of the community a better place, not out of some loyalty to Stack Exchange as a company or platform, but out of a desire to help the community I am a part of be a better place - both for asking and answering questions (on main site), and for making it a friendly and welcoming place for anyone who's part of the hobby (in chat).
That's not to say I don't have complaints about how things have played out. But one important reason I and other moderators have stayed on - or come back after suspending our activity - is because we care about our communities, even if the platform they're hosted on acts in ways we strongly disagree with. I assume many longtime users have had the same considerations, though obviously there's less "tying you down" to Stack Exchange if you're not a diamond moderator.
Obviously, everyone's threshold for where they draw the line - and what is just a step too far for them to bear - is different. But I think this is fundamentally the main thing you have to think about when you decide whether you want to continue contributing to sites on Stack Exchange.