On iOS 15.4 why can’t I change my cellular from LTE to 3G

The option is not present because your carrier has prevented it. Most carriers worldwide are in the process of shutting down their 3G networks, if they have not already done so. The presence or absence of options to disable LTE or 5G is entirely up to the carrier. They control what options appear on your device through profiles they distribute to devices on their network. Those profiles are specific to each device model and the software version installed on it. If your iPhone lacks the ability to force it down to 3G, that is because your carrier decided not to allow it on that particular device model. It is not uncommon for carriers to allow it on some older models but disallow it on newer ones that have better LTE radio hardware.