They get customers who are developers, who won't settle for closed source, or who want to self-host.

You could read up on the "open core" approach. Gitlab is a good example.

They can't compete with the closed-source ones. Open source is a very efficient user acquisition strategy. Not as much to turn users into paying customers tho. But it is a way to go from 0 to 1 in a market that otherwise would be very difficult to call attention.

A flip on the incentive might be that their core product uses open-source software somewhere in it and the style of open-source license may compel them legally to release any modifications, additions, etc. I don't know which licenses that may or may not apply to and IANAL.

Also for reference, RedHat's entire business model was like this, mostly charging for enterprise support. I use past tense because I have no idea what they do today. Way back in the day that mean when you submitted a support ticket, you might get a developer that worked on the product to help you if it was beyond their initial support triage level.

> What's the incentive?

More sales and trust. Vendors who open source are offering their customers the right to repair and self-host their products should they go belly up. That itself is a selling point.