In my experience maintenance is included only reluctantly. Much like any contract organization the owners of the companies make all their money on the margin between what they charge the customer and what they pay their laborers. For the laborers the upside tends to be high paying but comparatively short duration jobs within their chosen specialty. In my firm that largely sorts out to people between "permanent" positions or those who are semi-retired and don't want to work year round. On both sides of this equation you have people who generally prefer concentrated development (say a team of 3-5 people working 40-50 hour weeks for 6 months) than a longer duration but less intense maintenance effort (1-2 people working 10 hours a week for years).