No information - They rarely have any information (you have to go through several menus to find out your plane on most sites). They don't know when / where the plane was last serviced [0].
No framework - Your average consumer of air travel is not equipped to accurately evaluate that information should they even have it. (c.f., 'if it's not Boeing I a'int going [1]). They cannot make rational and informed decisions without information and a rational decision making framework
No control - They have no control over whether that plane is changed for operational reasons even if they have the information and a framework to evaluate it. You may not even know until you are onboard with the door shut unless you are really well informed about airplanes that the plane model has changed (e.g., I avoid A321s because
No recourse - Even if they consciously choose a plane based on information and an informed framework and then it gets changed what can you do? Are you even going to know until you get on the plane at which point what do you do? Do you think you get a refund?
It's also not guaranteed which plane will fly which route. Every airline has "substitution" rules in their conditions of carriage, and frequently use them.
