Also, the fact that HL2 needs so many gimmicks to add variety just proves that its basic shooting mechanics are lackluster.
Doom, Quake, and most of the Build engine games don't need a ton of gimmicks to work. All they need is good level design and you're good to go.
The thousands of user made maps for Doom, Quake, and the Build engine prove that. Most of them only use the core weapons and enemies and rearrange them in nicely designed levels. Some add more enemies and weapons, like Quake's Arcane Dimensions, but they still feel very vanilla and the gameplay is still about running through a complex hub-like level killing enemies with satisfying shooting mechanics.
Most of the popular HL2 mods focus on physics shenanigans or experimental stuff that plays around with the engine, rather than offering a straightforward shooty experience.
As an FPS, HL2 sucks compared to others in the field.