Perhaps in the US but it works fairly well in the UK.

Yes you may be sued (anybody may sue anybody for anything in the UK, Common Law. I have heard of a suit for the damage to a pair of jeans split up a seam by a first aider. It was laughed out of court) but the good Samaritan rules work well in cases of alleged negligence and cases are few and far between because of the high level of proof that is needed to have a successful suit also the costs are high so people are deterred unless there is evidence of extremely gross negligence.

Of course if you start to do silly things, like open chest heart massage by a car mechanic, you deserve all you get.

It is possible to buy insurance for workplace first aiders but I have never heard of anybody buying it and I was an examiner for the course for many years.

Does this help?
First aid insurance