How would you know ? And similar with insurance - how would you know they were correctly insured with "for hire" cover ?A lot of shoddy drivers have a licence but would you get in a cab of someone who didn't?
As you say, it comes down to assuming that someone else has done some diligence - you have to assume that the local minicab firm or hackney carriage licensing team have checked (Uber were accused of not doing proper checks on insurance at one point).
So it doesn't really matter whether we in here think the scams do a good job, the reality is that for most clients choosing a scam member is the easiest way to tick the "is the person competent ?" due diligence box. I think there's a consensus that being a scheme member isn't necessarily evidence of competence, but if the ---- hits the fan, the client can point to that membership and effectively move the spotlight onto the scheme for admitting someone not competent.
But all that doesn't mean someone who isn't a member can't get work - it just means they will need to do a bit more up front in providing different evidence to allow the client to tick that due diligence box.