Back in the late 1980s, when the Empire corridor trains were all unreserved, all of the trains except the Lake Shore were limited to a maximum of six cars. Many runs were covered by Turboliners that couldn't be made longer, but work rules were another factor. As I recall, for Amfleet trains, an additional crew member was needed if a seventh car was added. Fares between any city pair were the same on any train except Friday and Sunday afternoons. The Maple Leaf, as the first train of the day heading north/west from New York, was getting slammed on Saturday mornings, with people standing in the aisles to at least Hudson, 114 miles up the river, but Amtrak was reluctant to pay for the extra crew member needed to lengthen the train. Eventually they relented, and the Maple Leaf began showing up with nine cars. I am guessing this was the next increment beyond which yet another crew member would be needed. Since the switch to all-reserved seating and the advent of multiple fare buckets on each run, the Empire trains all went back to a maximum of six cars.
So, crew costs are a factor. But as others have noted, lack of equipment has been the major reason Amtrak consists seldom vary in length. Particularly on the long-haul routes, for the past 25 years they've barely had enough equipment to maintain the existing service levels and consists, so the company has instead focused on how to effectively ration the available space and maximize revenue from it.