Since having insurance is mandatory, if they all charge too much, they won't fail.

They do not have a monopoly. There is plenty of room for a handful of companies charging too much to fail. there is also plenty of room for companies charging too little to fail.

oh yes, please tell us about this imaginary perfect marketplace. where does it exist?

a perfect one doesn't, but you would be just as crazy to think that no market place exists either.

Insurance companies do in fact compete on rates.