This.
Sounds like you need to stop recruiting every temp that applies and actually have a proper hiring process to weed out the ones that are most likely to not perform.
If 1 out of 20 is decent then whoever decided to give them the job is quite simply rubbish at recruiting.
Part of the point of temp work like this (in many businesses especially warehouse, etc.) is avoiding going through too much time on the hiring process - often with group interviews, etc. 10 years ago you might hire say a group of 20 and over half of them wouldn't be a total waste of space - sadly these days that is often much less of a story.
Whenever I've done temping I was treated like ****. If I'd carried on doing it I would have ended up doing the absolute minimum as the norm, luckily my effort paid off and I was recommended for a perm role and haven't had to do it again.
This does have something to do with it - especially I wouldn't blame anyone for not doing more than the basics if hired on minimum wage with little else incentive wise - but more of the places I've worked for have had reasonable managers and conditions than haven't.