Many e-commerce startups fail because they make common mistakes like wasting resources, having an untrustworthy website design, providing duplicate product descriptions, failing to use responsive design, neglecting to target an audience, making SEO errors, complicating checkout processes, omitting information on about pages, offering poor customer service, and lacking a social media presence. The document recommends using ThemeJungle to build an e-commerce site to avoid these pitfalls and help businesses grow online.