I question whether it's actually true that they "aren't going to do it any safer than you would". I'd expect the typical person I hire to have a lot more experience than me which would give them a chance to be safer, to the point I'd expect it to overcome even the fact that they're probably not taking their time as much as I would.

Separately, being liable for something happening to someone else and that same thing happening to me are not equivalent. Macabre as it is, I'd rather pay out (or have my insurance pay) the lawsuit for a professional hired for a particular task ending up grievously injured than end up grievously injured myself. In a lot of cases, I'd doubt you'd even be financially liable. From a moral standpoint there's still some culpability, but in the case of garage springs I think your obligation is more to stress to the professional that you're happy for them to take their time and you're willing to pay accordingly than it is to subject yourself to the risk instead. From a societal standpoint, a few repairpeople building up expertise at a particular task is safer than everyone doing it individually.