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Also remember that id Software wrote an engine specific to the game they were creating (it's debatable whether you can call them engines at all, really). Things

Also remember that id Software wrote an engine specific to the game they were creating (it's debatable whether you can call them engines at all, really). Things are a lot more streamlined if you take this approach because you rarely, if ever, have to answer the question "would this work for some other title?". Or worse, "would this work for any future title that may not even be in the same genre?"! That, I think, is the mistake many game devs (who write their own engines) make. They make an engine for all possible games, then try to use it for their 2D match-3 puzzler, which uses about 5% of the code they've written, and even then they have to cludge some bits.

FRex 9y ago
They (id tech 2-4+) definitely are considered full on world class engines, FPS specific but still. They each have a renderer, physics, scripting, asset pipeline, map and data formats, tooling, etc. and were used to make other games. They are too big, unified, oppressive (as in - they control the main loop, their elements are all together, you can't mix and match easily, your game code is on a level of a plugin for the engine code to call, not the other way around) and multi domain to be considered libraries/frameworks.