How a typical business person thinks: "If I can take 1% of each of my 5 competitors' market share, I can make $x millions. Better head over to EDGAR and read some 10-K reports."
How uber-startups like Google and Facebook think: "If I can take the majority of all of my competitors market share through superior marketing and technology, and increase the size of the market at the same time, then we're worth $XX billions."
Notice how you don't have to invent anything completely new, just innovate on existing stuff that's proven to make money.