Low success rate of start up because many start up teams are weak.
I have many people coming to me with "ideas" for start up that flat out do not work. Their market research was almost zero and their ideas based on personal experiences rather than quantitative analysis. What's more, many aspiring start up founders have nothing to offer to the team: weak background in product management or tech, besides an unhealthy dose of nacisissm.
Over the years I have become more confident that a good team would probably succeed at a higher rate (25% if venture backed) than advertised(https://www.forbes.com/sites/forbestechcouncil/2018/05/30/seven-tips-to-increase-the-odds-of-tech-startup-success/) because of these impractical dreamers.