Most Americans get their health insurance through their employers. You are technically free to not sign up for this and purchase your own health insurance, but when you consider that you then give up the employer subsidy to your monthly premium, and that said subsidy often amounts to thousands of dollars each year, it does not make financial sense to participate in the market.
Most health insurance (and the lowest cost insurance) in the US is tied to a person's employer, a system that shifts the balance of power substantially to large corporate employers over workers and small companies.
Furthermore, absent major life events (job change, marriage, new child, etc) there is only one time of year when health insurance changes can be made, a time of year called Open Enrollment.
Finally, if you are self employed and you don't live in a state with a well functioning Obamacare market, your health insurance options are often quite limited.
Well, in the US, most people get insurance through their employer, who pays part of the cost. Switching would mean having to pay all the cost yourself, which would be economically painful.
And why doesn't the employer switch? Because health care plans typically have doctors that they like more than other doctors ("in network" vs "out of network"), so most people gravitate toward the preferred doctors. ("Like" means "cover better", so the patient pays less.) If your company changes health care insurers, then many people would have economic pressure to switch away from their current doctors, which is a hassle.
TL;DR: There's a lot of friction in various forms here. That's why. Yes, people can switch, but it's expensive and painful, so most don't, even though the option is technically there.
