One thing that isn't really mentioned but hugely important is the tooling - even if you can wire up various libraries for the subsystems of a game, you need tools for actually building the game. Stuff like an asset pipeline to ingest the numerous formats for different assets, optimise them for various use cases, a level editor, light mapping tools, etc. Those you can't get off the shelf, and make a huge difference when actually building gameplay and iterating.

In terms of wiring up various libraries, it's more difficult than it might sound initially because they all need to interact together, it's not so simple as just wiring up a few API calls together like you might do to build a web server.

Now of course CS 1.6 is built on the GoldSrc engine, which spawned from the Quake engine. Very different from modern engines, so obviously you can do it without Unity or Unreal or Godot. The question is if you want to, and if the time spent doing so is worth it.

People can and do build on top of Quake, so that could be a reasonable option if that's the feel you want for your game.