I think the issue is actually the reverse. Mountains are pretty common in Civ VI, so it wouldn't be too difficult to find a city that could build Machu Picchu. Rather, a mountain is an explicitly useless tile, for which there's no drawback to using it up for a wonder. The reason wonders in Civ VI are placed directly on the map is to make them compete with districts and improvements in terms of placement. Do you build a wonder, or do you save that tile for something else? With a mountain, that question is already answered, since there's nothing else you could put there.
That really isn't an issue in most cases. I often have 4-5 wonders in process of being built in 4-5 cities. Loosing 1 tile or even 5 to districts does not really impinge on the overall cities ability to grow or produce. The actual land it is on and near does, how you use that land etc.
Whilst I understand the claimed intention to force land to be an important factor.. you cannot farm or mine water.. but you can put certain wonders there, you cannot farm or mine snow or desert, but you can put districts and wonders there, some require it. There really is no feasible reason to not force specific wonders, esp those that actually got built on mountains.. to be placed on mountains.