Kayak.com gives you the option to sort and filter plane types! It's now one of their most popular features. Lots of people are happy to filter out 737-MAX planes and pay more for other flights.
The United app and website also show equipment type under the "details" expansion. You can also filter searches by equipment type. Yes, it can change but it's correct the vast majority of the time.
Add to that the fact that both, Boeing and Airbus, are fully booked for years to come, and it is pretty clear that the MAX isn't going anywhere as there are simply not enough alternatives to come by. Passengers aren't the ones deciding which planes are bought, airlines and leasing companies are.
There isba quite cynic name for passeneger: self-loading freight, also real freight doesn't complain.
"Passenger demand doesn't matter" is simply not true. Its effect is indirect, slowed, insulated, but it's still there. At least if sellers like Kayak enable differentiation by plane type.
It helps Kayak so, more people booking there by selecting a certain model. And the airlines, they can easily price tickets for non-737 MAX flights higher now (they propably don't, but why not?). In the end, it is pointless: planes get swapped out all the time, so there is no guarantee dor the model to be correct. Unless you fly with an airline that has no MAX's in their fleet. In which case this whole Kayak thing is on the same level like vegan vegetables or lactose-free cheese.