Actually shipping them turns out to be harder, but by that time you've got the money.

Yes, that tends to be how startups come off - even the "sophisticated" ones. They tend to have minimal knowledge of the steps necessary to turn their prototype into a mass produced thing: DFM/DFF, supply chain management, identifying/negotiating with component/bare board suppliers, etc. Generally speaking, they need lots of hand holding through the entire process, and that makes it take way longer for everyone.

The nature of stateside electronics manufacturing doesn't help startups much in that regard, in that shops are kinda either set up for NPI and rapid prototyping or not. Sierra Circuits is good for that, but idk how well they'd fit with a startup budget. Beyond that, the low-complexity nature of most IoT products means they're more cost effectively manufactured in China, as most US shops focus on low to mid volume runs of high complexity boards, as opposed to high volume low complexity runs. Figuring out how to manufacture in China can be a big obstacle for any company, especially smaller ones.