If everything is just kind of flat and grey, there's less noise, and it's easier to write A/B tests that produce meaningful results. That also means every product is flat and grey, though.
It could also just be a side-effect of having to appeal to every person on the planet, though. Sort of the design equivalent of a good politician, appealing to the lowest common denominator in all of us. Nothing to object to, but also nothing to love. Just flat and grey, with clearly displayed content and A/B optimized calls to action.
I find it incredibly boring and I'm getting tired of them rolling out new products that all look and feel exactly the same, but I suspect they have numbers that show any big design change would just be leaving money on the table.