Hubitat does not make money off the mobile app, they make money off the device and off subscription for mobile access. The app is a cost center.
Hubitat has a small dev team
There are a number of long-standing issues/bugs in the mobile app (broken presence being one) that perhaps the community could help resolve if it was open sourced
Above arguments also maybe apply for Web UI, but the mobile app has more urgent and persistent issues.
Hubitat is a community company, let the community help!