I'm not really tuned into the bike industry from the POV of the manufacturers.
Bike components seem to be pretty mature. The systems are, for the most part, interchangeable (stems, bars, saddles, etc...). I know, for example, that you can't mix/match campy and shimano shifting systems, but you can remove a Campy system and replace it with Shimano.
So, why the move to proprietary seatposts? As a heavy guy that has had several proprietary posts slip/tilt during training and racing, they simply don't work as well as the standard 27.2 posts. I won't even consider buying a frame that doesn't use 27.2.
I guess that there are some aero benefits. But those benefits do not outweigh the drama caused when the post has slipping/tilting issues or needs to be replaced.
27.2 posts, like the reference Thomsons, have an amazing track record. Also, if you break a 27.2 post, you just go down to your local shop and buy another.
So, can someone explain why bike manufacturers insist on creating their own custom (and often inferior) seatposts?