This is because startups are not supposed to be working on these "seemingly big" problems. If a problem looks big and looks like it potentially has a huge market, every other large company in the field would be already working on this. As a startup, it is stupid to try to tackle something that obviously big. That's why most successful startups start out as something that doesn't look threatening. Of course, there are genuinely ridiculous ones like "uber for clowns", "kickstarter for baristas", "airbnb for prostitutes", etc. but I can guarantee if you look back in a decade, what actually changed the world will have been what you now consider ridiculous.