The tech being there isn't helpful if it's too expensive to use at scale. And the computers then were... passable at best. It would be exceedingly hard to adequately control a rocket for an accurate powered landing at the time.

> you won't be doing four missions in parallel

Well you do have the problem of only having an ideal launch window every 2 years, so you do have to do them at the same time with minimal spacing or wait forever to try again. I think the plan was to only do two ships at a time initially though.

Some things you can compensate and plan for, some failures are just natural or random. Like a micrometeorite hitting two of the main computers or something.