Unless you have Critical Role's production team and budget, one's ability to represent terrain in miniature is vastly outstripped by one's ability to envision it.

Also, my current D&D character (in a campaign on hiatus until we finish Masks of Nyarlathotep) is an illusionist with mirage arcane and the Malleable Illusions subclass feature, and so if my DM tried to break out the terrain pieces, he'd quickly end up tearing his hair out when my character snapped her fingers and changed the whole battlefield. That's sort of an extreme example, but other large-scale terrain-shaping effects are common in the game and belong to the players; the DM has to not only possess all the terrain bits and models to model his own invented worlds, but also preempt the ways the players will change the environment according to their own will.