Because it targets all Sony, Nintendo and Microsoft consoles, OS X, iOS, tvOS, Windows desktop, UWP, Android and GNU/Linux. With binaries for every architecture being part of it.

Can target OpenGL, OpenGL ES, Metal, Vulkan, DirectX 11, DirectX 12, LibGMN, NVN.

Has graphical debuggers for shader development, a 2D GUI engine, a 3 engine with VR and AR support, asset pipelines for integration with studios workflow, and lots of features that Godot might one day have, but aren't on 3.0.

Kudos to Godot developers for what they achieved, I am also impressed with their work, but Unity devs aren't putting bloat just for fun.

In fact, any engine with similar laundry list of features has a similar size.