What is the most likely cause of short, quick, and useless reviews?
The basic idea is simple: reviewers have no incentive to write good reviews apart from their desire to see the conference succeed. Seeing an abundance of short, superficial reviews with a high acceptance rate indicates a low quality conference. 

An alternative explanation might be that this is a non-archival conference, often referred to as a workshop. In that case reviewers mostly care about the general problem the paper presents and whether it sounds interesting/relevant. Program chairs are usually those who go through the papers to see what fits (and there’s a small number of submissions).

If it’s not a workshop, the program committee/organizers aren’t well-respected researchers, and they charge you a lot of money in registration fees, I’d consider retracting.