But in practice, what I observe is that a number of witnesses are voting for witnesses with high vesting stakes, in the hope that their votes would be returned in favour. And they do return the favour. This leads to a situation where a circle of rich witnesses voting for each other.

... pretty much the same thing as if a miner in
Bitcoin buys more mining hardware than others. They have more vested
interest and would like to control as well as benefit more than others.
IMHO