All the tools exist to build your own. For example with a esp32-s3 and a custom pcb at one of the low priced fab houses in China. Sure you have to do it yourself.

The primary reason off the shelve products are cloud etc. is because these companies spent the time and money to do the above and since no on wants to pay 100+ for an iot switch they add cloud garbage etc. These products are now sold to the masses and if you have to support them you need control over them or your costs go through the roof.

I am working on a hardware iot product (no cloud) and I have to tape off the USB service port not because there is anything that could go wrong but because people don't read instructions and think the thing will power over USB when there is a power supply included with a barrel plug...