I'll answer for me, and 70% of Americans:

1. It was the only plan offered by my employer. Vision and dental insurance are typically optional extras, but medical is mandatory. Some jobs provide a choice between one or two different insurances, or a good/better/best plan. My wife (who is on a different plan than me, because she works elsewhere and neither of our jobs covers family members at a reasonable discount) selected the "better" option because she takes a few prescriptions and it works out to be cheaper long term.
2. N/A (but the answer is the affordable care act)
3. Insurance providers have a website where you can search for "in network doctors" by specialty. When I joined my current network, there were exactly two primary care doctors within 20 minutes of my home. Vision (which is separate insurance, generally) is often much easier to find. Dental (which again is separate insurance, often) is usually even harder to find a provider, in my experience. At my last job I could pay $200/month for insurance accepted by a dentist in my town, or $12/mo for dental insurance where the nearest dentist was over two hours away.
4. I would prefer the entire system was scrapped and replaced with government healthcare at low to no cost, including prescriptions, dental, vision, and mental health.