As another comment said, you’re referring to Verizon’s CDMA network. Which did not use GSM and this did not use SIM cards. Literally, no SIM cards. It wasn’t until their LTE network they supported SIM cards in any meaningful way. Their handsets were locked to their network and only worked on their network. Occasionally, an “international” phone would be sold that had CDMA + GSM so their customers could use it overseas but the SIM card slot did literally nothing here in the states since those phones wouldn’t have supported T-Mobile or AT&T’s network frequencies.

And just because nothing prevents a phone company from offering them, doesn’t mean they will. Having the ability to just move a SIM card is very pro-consumer in a way I don’t think esims will ever be (unless the EU regulates it).