If the minis came unassembled on sprues, it wouldn't cost that much extra to put variant parts on the sprues, so I wouldn't be surprised if we could have built multiple versions of a mech if Catalyst Games Labs wanted to sell kits, but it's clear that CGL wasn't to emphasize the gaming part of the hobby rather than the building part of the hobby by selling packs of preassembled minis.
Kinda sucks for the people who like building and painting minis, but it's probably better for the health of the game by having more mass appeal. I mentioned this before, but many people who are interested in various wargames aren't thrilled with having to build models first, and see it as a needless chore rather than a fun secondary hobby.
Now I WOULD love it if there were an option to buy both preassembled and unassembled minis (and if you're going preassembled, why not prepainted as well?), but that would add extra overhead of having many more SKUs (best case is double if kits were still sold as a pack of multiple mechs, but selling each mech separately would balloon the inventory management woes considerably)