There are lots of reasons on-demand services have taken off in the last few years -- smartphone adoption, mobile payment platforms, social authentication. But one that gets overlooked is a demographic shift in the workforce.

At the Code Conference on Wednesday morning, influential technology analyst Mary Meeker presented her annual Internet Trends report. This year, Meeker, a partner at the venture capital firm Kleiner, Perkins, Caufield & Byers, zoomed in on the peer-to-peer, "just-in-time" economy of companies like Uber, Airbnb and Instacart.

A major factor in the rise of these companies, Meeker said, is the arrival of the Millennials as the biggest generation in the workforce. Millennials' attitudes toward work makes on-demand jobs attractive to them in a way they might not be to older workers. 

Having entered the workforce during recession and slow-growth years, they regard jobs as scarce and don't expect them to provide long-term stability. According to surveys, they care more about flexible work hours than they do about more traditional benefits like paid vacation time, childcare or health insurance. More than one-third of them consider themselves freelancers, and 32 percent of them say they expect to be working mostly flexible hours in the future. 

They're also tech-savvy and prefer to work independently. Hiring managers acknowledge that preference as well, although they have a somewhat different interpretation of it: In a 2014 survey, only 27 percent of managers said they would describe Millennial employees as team players, versus 73 percent who would say that about Generation X members. Millennials had an advantage in "entrepreneurial attitude," 55 percent to 45 percent, but they also were four times as likely to be labeled narcissistic (80 percent versus 20 percent).