I think they have, though not as much as they should have.

I have a Sienna minivan that is hybrid and gets 36 MPG which is at least 10MPG more than everyone else using a beefed up version of the drivetrain the Prius created. (I can go 550-600 miles on a tank)

I also got RAV4 Prime which is a plugin electric that uses the same drivetrain tech and can go full electric, hybrid, or full power with both for over 300hp which is fun.

They achieve the best efficiency by using an Atkinson cycle engine which is more efficient but less powerful on the Sienna and Prius. What has confused me more is that things like the Grand Highlander still have a traditional cycle engine in their hybrid setup so you don't get the maximum advantage.