This makes sense because “the market” rather than a handful of cellular companies decided that people want LTE connectivity to be expensive and relegated to phones
Cell companies will sell connectivity to whoever will buy it though. Cars, watches, and anything IoT they can get their signal to.
For ~$30/mo in the US per device, hence why most people cannot justify owning multiple LTE-connected devices.
I think they would happily negotiate that price down to nil per month, if they could be absolutely sure a 'device' on your account wasn't actually your cousin 'sharing' your cellular account to save a bit of money.
Anecdotally, I've seen it closer to $10/month for car or smartwatch, and $0 for Kindle though I'm not entirely sure if they're still shipping cellular. Tablets are closer to that $20-$30, though.