I think Hardware is hard because when you put something out there, you can’t just send point-release via the app store to fix your issues/bugs. Hardware is much more permanent than Software, and it gets beat up and broken. The hurdles of maturity to ship a good product (and keep it sold — not generating tons of returns) and overall quality/durability are massive.
Look at Pokemon Go: buggy, crashes, etc. But scales so fast. Hardware cant do that. You ship something half-finished and your startup/company is toast. The cost of returns/replacement immediately wipes out your profits and you’re done.