Exactly. Gasoline, mains power, and batteries are standardized. So are LTE, 5G NR, and Wi-Fi. Compare what Mike Masnick of Techdirt and other Internet user freedom advocates have called "protocols, not platforms."
Though even if there were no cryptographic lockdown of these "smart" devices' system software to interact only with the vendor's server, one big obstacle to running your own server (with proverbial blackjack and hookers) is that so many Internet providers nowadays block inbound TCP connections. T-Mobile Home Internet, for example, puts subscribers behind carrier-grade network address translation (CGNAT) with the whole neighborhood behind one IPv4 address. Even through IPv6, their gateway appliance offers no port forwarding or DMZ option.