Publishers are not a unified group of people that work together, if they did it would be illegal, so your last statement is meaningless. Secondly, the people buying habits kill the middle shelf games, publishers are going to make what sells not what a fringe fraction of the population buys. Thirdly, player expectation of games significantly increase, players want fancy graphics, high quality voice acting and all the bells and whistles, which acts like a barrier of entry to many middle shelf developers.
When a middle shelf developer fails to match the quality of provided by AAA reviewers end up destroying and trashing the game and nobody buys it.
Finally, not every game can be minecraft, just like not every game can be WOW. Minecraft is a very unique idea at its time that thanks to a combination of luck got bigger than its creator had imagined.
This is not unique to the video game industry. Most disruptive innovations (think Netflix, facebook, snap cat, oculus rift) are done by entrepreneurs, while larger companies are focused on sustaining innovation (think ps4, it is simply an improvement of ps3). Eventually the larger companies find value in the entrepreneur's creation and buys them out, example facebook buying oculus rift.
The point being it really isn't unique to gaming, it happens in EVERY industry.