It's a combination of a) founders/ceos not being aware of alternatives, b) many perceived alternatives don't in fact offer an alternative, but a layer on top (zenefits). and c) since ADP is so difficult, the pain of getting off of them - and the perceived pain of switching to someone new, is a large enough disincentive to keep folks from switching.
New companies like Justworks don't build on top of other systems but have built everything in house. You don't need to coordinate with payroll providers, or health insurance brokers, or even government agencies (to do your W2s, I9s, 1099s, etc) - because they literally do it all for you.
On top of that, their software doesn't look like crap and their support staff is actually, you know, supportive. Oh, and they're not "Free" like Zenefits- but they save you so much on healthcare you end up spending less than you would on "Free" software that doesn't work.