It's a stereotype that startups don't have things like estimates of revenue, a clear business plan, clients, etc... A few crazy outliers get all the attention but the vast majority that get funding have a clear and convincing plan to get to profitability, and often clients or at least partner businesses (in other words clients that aren't paying yet, but are willing to spend their own resources working with you).
The problem is that most new businesses fail, so if you're investing in new businesses the winners can't just make you a little money, they have to pay for multiple losers too.
You also need to convince investors that it's worth putting their money into these risky businesses instead of say Microsoft/Apple/Google/Amazon/etc... which will not go out of businesses anytime soon and produce respectable returns.