From what I understand, a market maker buys at bid price and sells at ask price, whereas the price taker involved with the market maker buys at ask price and sells at bid price. Basically, the market maker pockets the bid-ask spread.
In this case, shouldn't buyers and sellers try to meet half-way, irrespective of volume or other measures of liquidity? Doing so would ensure that they pay the minimum amount, right? Of course, that would be a problem for the market maker who wouldn't be making any money anymore, but from the point of view of the price takers, why do they not strive to meet half-way?
Alternatively, why is the de facto business model of market makers to pocket precisely the bid-ask spread instead of, say, a flat fee?