I don’t think i for one understand. Infite maps are about infite as your computer allows them to be. The space can be defined as infite(x,y,z) but even if you’d have something simple as pointer to your own location you couldn’t really travel in this infite space forever because eventually you’d have your computer RAM filled with your location in the space.(x: 9999999999999999999999999999… and so on, ya i know you could multiply it with dimensions but its a crude example of simple space).
From what i’ve understood Source engine limits the map size and the amount of objects, I’m not sure why but i’d expect its probably to prevent lag. Yeah and of course because of the design of the engine, If you’re going to precatche the map you need to reserve memory for it because directly loading it would take too long considering the objects are complex and there’s no real way to predict the object properties effectively. You also have these complex map shapes and other properties which you need to load into the memory inorder to make the main calculations faster.
In minecraft you got very simple objects and basically no physics. So you’re saving a lot of computing power right here and your CPU will process it far more effectively because it’s a much more simplier enviroment.