Mostly because developing for iOS and macOS is terrible, especially when your app needs to have "entitlements". Tailscale uses a "Network Extension entitlement" which is linked to our corporate Apple account. Even onboarding new employees and getting them up to speed on xcode/macOS/iOS development is painful. It often requires a bunch of messing around with Keychain and random reboots (not just Xcode restarts!) because something in the macOS kernel gets confused. For some development we also need to disable System Integrity Protection. And make sure there aren't duplicate copies of certain files between /Applications and ~/Library/Developer/whatever.
And then once you get it all working, some cert or login or something in Xcode or Keychain expires in a few months and you have to re-learn the whole esoteric dance once again.
The whole process of developing Network Extensions is pretty terrible.
Even if we open sourced it, you couldn't just git clone it & hit play in Xcode. Even if you paid Apple $100/year, you still couldn't, because your Apple account isn't blessed enough with the right to use a Network Extension.
It's hard enough for us to support Apple platform development internally without helping the world learn Xcode/code signing/entitlements/Keychain.
I've been and remain a huge open source fanboy for about 25 years now. If I thought we or the community would benefit from it being open source, I'd argue for us open sourcing it. But it just doesn't seem worthwhile. Or maybe I'm just still angry at the platform.