Disclaimer: I am currently building my first product and have no idea how any of this works.

In this day and age where almost every service you’ll ever need in tech offers a free tier and is reasonably affordable for scale, why do startups need funding?

For example, I am a relatively talented full-stack dev, if I were to build a website/app I can do it all by myself in a reasonable amount of time. I do understand that this is not the case for everyone, but for those that it is, why can’t they do this:
Build a product -> get a few users -> (make money from those sales ->) put that money into marketing -> get more users -> more money -> bigger marketing -> repeat

I’m more surprised when I read that a SaaS received funding because they usually don’t need a huge research and development team that a company like OpenAI would need nor do they depend on ad revenue which pays cents.