And if you've ever had a mechanical linkage fail, or noticed that wiper linkages only work for certain geometries, you appreciate the reliability and versatility that comes from having separate motors.

Also, one wiper arm can be completely jammed or disabled and the other still works, where a mechanical linkage would be down for the count.

Cost will come down. Power windows used to be a luxury item too, now they're ubiquitous because the hand-crank mechanism is not only more expensive to produce, it also placed constraints on door panel styling, and having a single door design is cheaper overall.

When the hood and windshield don't have to accommodate a wiper linkage, engine packaging gets easier, allowing designs that could be safer, cheaper, more efficient, more reliable, or some combination thereof.

These are built for a reason. CANbus means they share the wires with lots of other components. Wiring and the complexity in the other end of that wire have a cost too.

Mechanical solutions often weigh more, as well. We're supposed to be designing for better gas mileage, aren't we?

Good gas mileage is one of the many design goals. Another important one is reliability.

and theoretically a computer-driven windshield wiper would be able to respond to weather conditions better than a purely mechanical one, improving safety (perhaps marginally, but that's ok by me). I really appreciate my auto-rain-sensing windshield wipers.

As I said complex behaviour would benefit from computing. I don't see car wipers having complex behaviour that they would need computing to implement it. Also the complexity of the behaviour is a spectrum and not just a on or off thing. You need to consider all the various properties to figure out the best balance in the solution you come up with. On the industry side of things, companies will fit everything with computer and sell it at higher cost coz thats what their goal is i.e to make more and more money, they don't care about the best solution, all they care about is how they can sell expensive solutions and use proper advertisement to portray their solution as the best one (even though it is not).

Many modern cars are equipped with rain sensors that auto adjust the speed and frequency of the wipers, and they seamlessly move from one setting to the other instead of having distinct steps or settings like wipers of old.

I can see why a programmable wiper motor is useful that scenario; send the speed and frequency as arguments and have the motor adjust on the fly.

It's probably not about the complexity of the wipers, it's about the complexity of the wiring system. By putting everything on a bus you only need to run one set of wires around the entire car, rather than individual control wires to each gizmo.

With multiple wipers and no mechanical linkage, preventing blade collisions becomes a concern.