I understand what you're saying, but as someone that has followed various game developers closely, especially those that do porting work, source access is a huge win in many cases.
Imagine your game shipped on a version of Unity that's no longer supported but you want to continue using it for projects or bring back an old game to a new OS platform. Without the source, your only option is to port the game to a new version of Unity. That's not so great.
But again, I'll readily admit that all of this is only a win if you have the right team with the right skillset. There are definitely tradeoffs involved.
I think at scale, that's true. But what if you're a small team, or a solo developer? Let's face it: there's no way you'll be able to port Unity or Unreal to a new platform by yourself, so that might as well not even factor into it.
I think Ryan Gordon (aka 'icculus') is proof that the right solo person can do such a port :-) Nevermind Casey Muratori or others I could name...
There are many small, talented indie developers that definitely have the right set of skills to bring an engine to a new platform, or to one that's just slightly different.
For example, porting an engine from Linux to FreeBSD or some other UNIX should be trivial, but impossible without source code access.
Now I doubt many games are going to be ported to those platforms, but imagine a 3D walkthrough program for an architectural client or manufacturing facilities where UNIX is still found.
Imagine your game shipped on a version of Unity that's no longer supported but you want to continue using it for projects or bring back an old game to a new OS platform. Without the source, your only option is to port the game to a new version of Unity. That's not so great.
But again, I'll readily admit that all of this is only a win if you have the right team with the right skillset. There are definitely tradeoffs involved.