Startups are not (usually) engineering firms with super high quality requirements. My first job (still in college) was at a startup. I was the only developer under the CIO. It was a java webapp and both of us barely knew Java when we started. It worked pretty well, we had minimal serious quality issues compared to most companies I've worked at, we had zero unit tests and a shoe-string budget. That company is still going strong and has made millions off the project I started practically on my own over 10 years ago.
It can be argued that startups require a broad range of skills in a wide array of technologies, and you don't get that with inexperienced people.
In a mid-sized organization you can work on a java webapp - as in your example - and possibly only on a part of that java webapp. In early stages of a startup, you're likely to have to know your way around the webapp's frontend, backend, mobile app/mobile web adaptation, handle SEO, plan for scaling, be a part DBA, handle server administration DevOps style, and do all kinds of internal business process automation unrelated to that product you're building. You'd need to be a jack of all trades at beginning - since there's noone around who's specializing in that other stuff - or spend half of your time becoming a jack of all trades.
I installed the servers in the rack at the data center, configured the load balancers, installed and configured the database with a hot backup and wrote 90%+ of the application code when we went live. You don't need people with all that much experience.
