What's the incentive for individual sites or browsers to do this?

From the site's perspective, they're going to need to have a WebPKI certificate for the foreseeable future, basically until there is no appreciable population of WebPKI-only clients, which is years in the future. So DANE is strictly more work.

From the browser's perspective, very few sites actually support DANE, and the current situation is satisfactory, so why go to any additional effort?

In order for technologies to get wide deployment, they usually need to be valuable to individual ecosystem actors at the margin, i.e., they have to get value by deploying them today. Even stipulating that an eventual DANE-only system is better, it doesn't provide any benefit in the near term, so it's very hard to get deployment.

A fun note: I vibecoded a dumb thingy that monitors the top 1000 zones on the Tranco research list of popular zones for DNSSEC status:

https://dnssecmenot.fly.dev/

Obviously, the headline is that just 2% of the top 100 zones are signed (thanks to Cloudflare). But the funnier thing is: in 5+ months of letting this thing run, it's picked up just three changes to DNSSEC status among all the zones it monitors. The third happened just an hour or so ago, when Canva disabled DNSSEC.