For many products distribution is correlated with marketing and networking, this isn't true for all products (e.g. web-apps).
You still need a distribution channel. That requires either capital or connections.
Just putting something up on the Internet and hoping will get you nowhere.
Of course it's possible to get extremely good ROI on free advertising (especially if your product is excellent or unique), but it's the exception, not the rule.
You are right that free advertising requires work like engaging in the community, but all startups require a considerable amount of work, and if you have a little bit of online savviness engaging with a community is very easy.
I know that from the time I went from $0 to $10,000 MRR I spent about $100 in online advertising, and I know countless other businesses that have done the same. Maybe if your plan is to create the next Facebook it is harder, but if you have a high quality niche startup, being able to effectively and cheaply advertise yourself for free is hardly an exception. You can always broaden your scope later.
Also, the markets that require the least capital/connections tend to be low margin and flooded with competition.
Now, you usually would need to work this out in advance with the investor. After they've looked over your deal and poked at you some, tell them that you are going to go get a big customer with the permission to declare that funding is lined up pending contract.
It doesn't seem to take much more time out of your day than doing one after the other, and often the investor will get so distracted by the large pending customer that the due-diligence process becomes a pretty cursory check-box affair.
Software is still made by people, and people have basic needs, and if you don't come from a family with sufficient wealth to support you, you have to work your way through.
In the USA, you have to do that in a nation that does not open doors for you based on birth, despite the myth the feudalist-owned media perpetuates. This is a feudalist society, the most feudalist society ever known to man (at least in the preserved history).
My grandfather (and father) were that person.