That's a bit of a generalization. Mechanical keyboards like the Hyper7 exist that have enough for even the most key-hungry typist. If you want an ergonomic mechanical keyboard with many keys there are also some of those out there, like the Kinesis line or the Glove80. That said, one thing people mean by ‘ergonomic’ is ‘less stretching’, and by geometry more keys = more stretching, which is why there's significant overlap between the ergonomic keyboard crowd and the few-keys crowd. When I first got an Ergodox I too thought I would struggle with few keys, but over time I found I prefer to have the keys in a comfortable position with a layer modifier than to have them off in Narnia where I have to shift my whole wrist to reach them.
(Because subcultures are subdivided recursively, the _gaming_ mechanical keyboard subcrowd also often like small keyboards, for reasons I don't really know but are presumably quite different from the reasons the ergonomic mechanical keyboard subcrowd have.)