The reasoning is very simple, it's efficient. Government mandates fuel economy standards. Every manufactuer has to have meet for fuel economy. They average the MPG of each individual vehicle that is built at it has to average out to the standard or higher. It takes a lot of TDI jettas to help VAG average out the Bugattis. For every big power gas guzzler a brand produces it has to have enough super efficient vehicles built to keep te average in check. That standard jumps to 35.5 mpg average for the 2016 model year and to an astonishingly high 54.5 miles per gallon for 2025.
Technology in cars traditionally has been on a pretty slow but steady climb and recently it's freaking skyrocketed. Look at some of the technology Audi uses. TDI clean diesel technology, cylinder on demand in the 4.0 shuts down unneeded cylinders and uses AVS (adjustable valve lift) to basically shift to a roun cam profile so energy isn't wasted opening and closing valves on cylinders that aren't being fired, pretty much everything has multi stage oil pumps because it's more efficient, the 4.0 twin turbo has inside out heads with te exhaust and the turbos in the center of the V because it's more direct and efficient, there is an electric turbo coming out that with have exhaust and a generator attached to it, every letter series is getting a plug in hybrid, etc etc etc.....
10+ speed transmissions are coming quickly and are nowhere near the end. The next decade of automotive technology is going to blow your minds.