They're just not efficient or convenient. Like all public transport, you have to run on their schedule, not your own. You have to deal with their limitations. They're useless for going to the grocery store and bringing home a dozen bags of groceries. They're useless for going to the hardware store and bringing home 10 sheets of plywood. They're useless if you've got to go when and where you've got to go.
My daughter, who now lives just outside of DC and takes the subway most places has started to realize that.