I think the high-risk, low-reward explanation of the exclusion of earthquakes (and tsunamis) isn't a great one, especially considering they could relatively easily make earthquakes a feature that can be turned off.
Additionally, as others have alluded to, there could be attractive reasons for living near a fault line, ones that Firaxis could have played up if they felt there was not enough reward. So to say that there is no possible benefit to earthquakes is a bit unimaginative, and not really tied to reality imo.
Also, with increasingly advanced technology, humans have become more and more equipped to handle and prepare for earthquakes. There is something beautiful to colonizing an earthquake zone (or tsunami-zone) and figuring out how to deal with it / make the most out of it.
Lastly, and most importantly, Civilization 6 is a thematic game. Excluding certain events simply because they are inconvenient can take away immersion. There are always game and gameplay design ways to mitigate any frustrations that come with random, punishing events. For time immemorial, people have settled, and continue to settle on fault lines (and tsunami-zones), for whatever reason. Many of the most thriving metropolis' in the world have and continue to exist in these danger areas.
I think enough people will be puzzled by such an exclusion that they may put it in a future update, if it's not in this expansion.
IMHO, the reason earthquakes have been neglected is they are not that climate-related as storms, flooding or rising sea levels. However, volcanoes are not related either, but maybe they are more iconic (and, in some way, they overlap with earthquakes: both are tectonics-related and would appear, more or less, in the same zones).
That said, there are quite good point fors made in this thread for considering the earthquakes: predictable and risk-reward choice as the chance of them appearing is located in the (rich) hilly zones in continental divides. Benefit could be provided, not in additional food, but additional production (or even resources) due to mineral deposits reveal. Effect is clear and similar to other disasters: damaged units, pillaged buildings, disctricts and improvements in the earthquake zone.
Volcanoes are very climate related.
In the end what has to drive the game is the fact that it is a game, not that it is a perfect recreation of real life, and adding in earthquakes might not fit into that paradigm
Theme and immersion are very central aspects to most games. Also, there is a reason why Civ falls under simulation genre.
If earthquakes are in, gameplay-wise they will be very similar to storms - randomly occurring disaster that will hit a random small area (maybe earthquake is more common on continental borders, but exact location is still random), causing damage to units and infrastructure. It is difficult to give earthquakes a unique "flavor" without too much overlap in functionality.
The same could be said of having more than one of any disaster then. Storms are also much more predictable than earthquakes as we know it, and neither are completely random.