One of the reasons is probably because parallax on mobile is a horrendous experience and people have started to catch on to this.

I've only seen a handful of companies who can pull it off well. This is probably one of five I feel does it well: http://www.extreme-sensio.com/

That's because it is simply done. For the most part the only thing that really happens in a big way is that the background changes out based on scroll position. Any other element that animates on scroll does so in a minor and subtle way. The actual scrolling of the page is not altered in any way.

I agree, this is one of the better ways to handle such things if it has to be done.

I tried to buy a theme from themeforest and it's extremely hard to fine one that is NOT completely polluted with CSS animations and parallax effects. I wish you were right, but just from the top-selling themes of marketplaces like themeforest, it seems that the trend is still quite big, unfortunately.