Uber does have a carpool option. But I'm not sure how often it gets used.

I don't understand how anyone ever thought they could reduce traffic. Even if they only served people who would otherwise have driven, a cab replacing an A to B and a C to D journey has to do three journeys to replace those two (A to B, B to C, and C to D). It was always going to increase traffic.

The text of the article explains that it's based on reducing the number of taxis (or cars for hire generally) on the road, reducing parking spots, and increasing carpooling:

In the 2010s, the Senseable City Lab at the Massachusetts Institute of Technology, where one of us serves as the director, was at the forefront of using Big Data to study how ride-hailing and ride-sharing could make our streets cleaner and more efficient. The findings appeared to be astonishing: With minimal delays to passengers, we could match riders and reduce the size of New York City taxi fleets by 40%. More people could get around in fewer cars for less money. We could reduce car ownership, and free up curbs and parking lots for new uses.

This utopian vision was not only compelling but within reach. After publishing our results, we started the first collaboration between MIT and Uber to research a then-new product: Uber Pool (now rebranded UberX Share), a service that allows riders to share cars when heading to similar destinations for a lower cost.

It goes on to explain that it's a problem of induced demand (same phenomenon that causes highway expansion not to actually help with congestion in the long term):

Alas, there is no such thing as a free lunch.

Our research was technically right, but we had not taken into account changes in human behavior. Cars are more convenient and comfortable than walking, buses and subways — and that is why they are so popular. Make them even cheaper through ride-sharing and people are coaxed away from those other forms of transit.

This dynamic became clear in the data a few years later: On average, ride-hailing trips generated far more traffic and 69% more carbon dioxide than the trips they displaced.

We were proud of our contribution to ride-sharing but dismayed to see the results of a 2018 study that found that Uber Pool was so cheap it increased overall city travel: For every mile of personal driving it removed, it added 2.6 miles of people who otherwise would have taken another mode of transportation.

Not only that, it also takes passengers away from public transit because door to door is more convenient than waiting for a bus or changing lines in between.

Idea is that instead of 4 cars containing 1 person in each of them you get 1 car with 4 people in it. No idea how well it works in practice though, I assume most people who already drive will want to keep driving alone even if it is more expensive.

You've never heard about capitalism? Zero labor cost means it's cheaper to have 100 taxis in your fleet when you would normally have 10.

If anything, I see it becoming the board game Othello to a degree, the big companies flood every inch of road with their cars instead of the other guys. I'd even see them using groups of their robo cars to create intentional traffic for their competitors, only to then communicate back to their own fleet where the only viable route through town is. This way it's like a tooth eat and if you want to get across town, you know it will take you 15 mins with Y brand and an hour plus with A brand.

Wake up and smell the death march called endless corporate growth.