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It’s the design methodology and optimisation of the source engine. If you pick a source engine triple A game, it uses mainly BSP and meshes for decoration. If you take an unreal engine (3,4) triple A game, it uses mainly meshes.
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It’s true that idTech rendering is specifically built with in mind and thus optimized for it. However, I think the point that being made is that the style of building with tools that handle level design-specific needs is a huge benefit for level designer iteration and thus potentially for the development of the game.
Even if on the back end the engine were to convert everything in to meshes (as I understand it the HammUEr tool does) the greater benefit that can be realized is the improvement on human interface/development time, not necessarily some rendering/compilation need. I have gotten to the point with the existing tools in UE4 where I can block out levels relatively quickly, but I would see a huge benefit from some expansion of that toolset and cleaning up some of the weird collision issues it causes.