[drb91]

Pretty sure Unity is more heavyweight than a few hundred kilobytes of a library.

It also doesn't come installed on my machine.

[pjmlp]

Depending on the project type, the final release package can be just a few hundred KB, like they do when targeting WebAssembly.

OpenGL also does not come installed on my machine.

[drb91]

Right, but it still involves several gigabytes of largely unused functionality to get a “hello world” and it pigeonholes you into a specific ecosystem. Unity is an entirely different offering from a graphics api.