Programmers in successful startups (like the ones producing the demos you mention) ignore what people tell them that they can't do. They just try and they get something done. Then they iterate as long as "the demo's ui sucks" is the number one problem.

Probably the designs are sourced from their experiences. Just like you, many of them have seen beautiful sites. It's not impossible to find a few you like and mix/match design elements plus some of your own originality to produce your own variant.