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.
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.