The reason: huge textures, where every detail is unique and that look good up close. Let's say you map the world at a millimeter scale, let's a byte per mm2, 1GB is 1000m2, which is about the size of a typical backyard, small maps are bigger than that, have a bunch of them and you can get to tens of GB easily. Plus textures are not just colors. Normals, material properties, etc... are mapped to textures too.

I don't know how Counter-Strike 2 gets to 85GB, but if you want things to look good up close and avoid repetition you get these sizes. Also, storage is cheap, so if it makes the game look better, why not use it?