Some of it comes from some de-facto standardization on boostrap - or at least some libraries that look an awful lot like it. It goes hand-in-hand with the need for responsive design: the header, splash image, and columns can collapse into a single, thinner column readily, so little thought has to be put into it.
A long while ago I was noticing how certain sites had a certain "professional" look, and was trying to put my finger on exactly what it was. i came up with:
- Nuances that move beyond just colored blocks: gradients, pinstripes, rounded edges. A lot of these are actually pains to pull off! And it probably does take some legitimate skill to do well.
- Use of stock photography... people just sittin' around, looking like they're lives are somehow better with your product
- There are certain layouts that are becoming defacto standards... all 3 of these sites use layouts based on a TV or billboard, with everything appearing above the fold. And no-one seems to use "flow layout", in general fixed columns are considered a better idea.
Plus ça change, plus c'est la même chose. Except it doesn't take that much skill to pull off now.