Several reasons:
1. their car can't go that fast without too much strain (55-65 is a reasonable expectation of all highway vehicles, but some states with 75+ limits is pretty redonkulous)
2. more fuel efficient - on some rare occasions I've dropped down in speed and drafted behind semis @ ~55 in a 65+ zone in order to increase my chances of making it to a gas station while on road trips
3. they're not a speeding douchenozzle. Speed limit is supposed to be the maximum, not the minimum or average. I'm not sure what the laws are in all the states but here its anything above 5mph over the limit and you automatically qualify for infractions against your license (if you get caught)
4. you complaint doesn't hold any value unless these drivers are going that slow in the faster passing lanes. You have no right to complain unless these drivers are blocking your ability to pass them. I can understand it on some 50mph non-highway roads where you don't always have a passing lane and people treat them like they're a common 35mph road, but on the bigger highways with passing lanes you really sound like a dbag.
At any rate, traffic would be 10x better at least if everyone used cruise control on highways, I hate getting passed by some rubber-banding retard only to have to immediately get over to pass him as he merges back into my lane and slows down, to which he immediately speeds back up because how dare I pass him up, lol
You drive what is a safe speed for the conditions, not whats based on the maximum.
keep in mind that includes keeping pace with traffic around you, you can get a ticket for driving too slow on a highway