It's simple economies of scale. Many receivers are made, few pre/pros. It always costs more per unit if you don't make many of them. For example, there are fixed overhead costs (like paying all the engineers involved in designing it). If there are fewer units made, a larger fraction of that fixed cost has to be earned back by each unit sold.
In other words, people buy pre/pros for reasons other than price.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped