Sails take quite a bit of space to store, they aren't exactly the lightest thing, and you need training and coordination of the sailors to deal with them. They also probably just dont provide enough benefit to offset the costs -- as I said before, sails don't actually provide that much force.

There are a lot of smaller boats that have both sail and mechanical engines, but that only works because they're small enough that the sails can provide a significant amount of force. And if you look at the transition period between the Age of Sail and Age of Steam, there were a lot of boats and ships that had both sail and steam engines. The sails were mainly relegated to backup, or to help out the steam engines, but these hybrids existed.

I just looked into the economics of rotor sails.

On average, they save $400/day ($146,000/year) in fuel costs (2017) or about ~10% of fuel consumption. In real terms, it's not much.

They cost about €1-2 mil ($1.1-2.3). Under the most ideal circumstances, that is a ~8 year payback, which is fairly long.

In order for the economics to work out, oil prices need to be higher and externalities (CO2 emissions) need to be included in the price.

We do. They just don't look like sails. This is a Maersk ship.

https://en.wikipedia.org/wiki/Rotor_ship#/media/File:Norsepo...

Low tech magazine has some practical examples of ships that operate sails in addition to fuel to lower fuel-usage: https://www.lowtechmagazine.com/2007/09/sailing-ship-re.html

At 10 mph wind, sails generate about 0.256 pounds of force per square foot (according to random website found via google). Therefore your several ton cargo ship gets next to nothing from sails.

We use the form factor quite differently. A game and watch is about the size of a cell phone these days.