Unity sells source code access, it's in the "contact us for quote" category. But that's how many B2B services work, including some of the most widely used commercial game middleware.

Also large portion of first party Unity subsystems which you install through builtin package manger (I am not talking about third party asset store stuff) is available in source code form under relatively nonrestrictive Unity Companion License to anyone. Don't need the Enterprise plan or additional payments for that. For those modules you can not only read and modify the source code, but you can even openly distribute your modified versions. The biggest restriction of this license is that you can only use that source code in combination with Unity, you can't port it to different engines. And in many cases where I read it, to better understand how to correctly use the library or avoid a bug, it was quite readable.

So overall access to source code isn't the main obstacle for game developers to fix the problem in Unity, spending time fixing things and afterwards maintaining a modified version is.

I have personally had issues where I wanted to read the Unity source code but couldn't. And I suspect it wouldn't have been worth the cost in my case. There's really no substitute for shared source. Truly open source isn't necessarily better; modifying it is not often realistic, but having to pay just to read it is a big stumbling block.