First thing to note is that machetes are often sharpened with files, and so end up with a very coarse, microserrated edge.
More importantly when thinking about machetes and serrations, generally speaking, serrations can assist with slicing, but hinder push-cuts. Now, think about machetes are often used for -- bushwhacking, clearing trails or campsites, small de-limbing jobs, sometimes coarse food prep, etc. Very little of that is heavy on slicing. In fact, often we don't want to feel "bite" at all. With a machete, the main point is often to get the blade through the material in a swift single cut, not to bite and slice. If I'm delimbing small limbs, I want the machete to go all the way through the limb with a single stroke -- serrations don't help with that. If I'm trying to cut through tough small branches and grasses, serrations grab and pull, which is precisely what I don't want -- I want the blade to go completely through the twigs instead of just bending it them one way, and having them snap back.