My guess is that they want packages on the Unity store to be usable on all platforms they support.

For example, if you build an app for iOS or visionOS, you have to distribute it via Apple’s stores, and there are technical barriers (code signing, DRM) that get in the way of an end user being able to exercise their right to replace LGPL components with modified versions. I mention this example because there was a big tie up for Unity and visionOS announced with the Apple Vision Pro last summer, but presumably the same applies to Unity games on consoles etc as well.

I can see why from Unity’s perspective it is cleaner to just forbid LGPL entirely, that way it is harder for their customers to mistakenly violate licenses.