The largest private company in the world, Walmart, uses Microsoft software from their servers down to CE on their Telxons. I suspect you are confusing primarily IT companies with companies more generally.
I'm not saying big companies don't buy software. What am saying is the process is very long and it's better if you are a big established company selling to those big companies. This by definition is not a startup.
As a startup you could spend a whole year with everyone working on a deal with a big company and have it fall through - then what do you do?
