I believe I always contributed, but it was minimal for a decade and change. I think there are a few things at play:
1. When you're young, retirement doesn't even seem real. And depending how much dough you're making (or not) when you starting out, your first instinct is to try to keep as much money close as possible. Unless you have someone in your ear that can explain how advantageous retirement accounts/savings can be, folks tend to view that money as just an extra expense instead of what it can become.
2. I didn't realize how many folks have no idea what is actually IN their retirement accounts. Some think it's just a bank account or magic money savings machine, not realizing that there are actual businesses behind these funds, that will compound your money backed by the machine that is the American Economy (VTI/S&P, at least).
When I finally put that in perspective (way too late), that's what really turned me on to things. At a base level, in my mind, this whole investing thing is the easiest way to actually become a business owner w/o the bullshit and friction. As long as you're patient it really can be a magic money machine and grow whatever money you DO have.
Like Marks says the key is to BE invested.