This. Some of the custom maps are much busier and none are nearly as well optimised as the official maps.
Additionally there's level size, geometry complexity, textures, shaders other resources, etc. Just because it's the same engine doesn't mean you can throw just any map at it and the reqs will be the same. You could easily bring down even high-end PCs if you wanted to.
I did some minor dabbling with mods/maps during the Quake-days, and also had a small go at HL1/Worldcraft when the game shipped. Being based on the same engine, the mapping process, tools and restrictions were very similar, and there was an insane amount of tricks you could apply in the editor to make things run better. Everything from scaling textures on "invisible" surfaces, converting solids to entities so they didn't factor into BSP computations, to moving hallways just a few units, could have major impact on your level's framerate. To get optimal results, as Mr. Faliszek says, "you have to know the engine and the tools very well". While there is tonnes of knowledge in the modding community.......
Nobody has the all of the creative talent, technical skills, technical resources, time available and QA that would be applied to an official release. Or for that matter the inclination to do the most boring things like plan.
Then again, no amount of optimization can make the aging console hardware catch up with PC specs. This is probably going to be more about compromise than optimization. The mod-makers' main incentive for expending the effort will be having more people able to see their work I suppose, which is pretty strong as the drive behind most non-profit projects tends to be "for fun&glory".