Manufactured or pre-made split keyboards aren't cheap. However if you're making your own the costs aren't all that different. So it's easier to ignore the cost and effort if you're making a split keyboard. Also by definition, you're talking about someone willing to go down the less beaten paths.

I would love for split keyboard to become more mainstream. They're far superior to the traditional key layout. But if the sample set is customs, it's probably selection bias.

The board is less expensive to fabricate because it is physically smaller.