why doesn't my apple watch show the option for cellular

If the Digital Crown DOES NOT have a Red Ring ⭕️ around the face of it, then it does not have a cellular radio.

If it DOES have the Red ⭕️, then it does have a cellular radio, and it should give you the option of using your iPhone's cellular phone number.

NOTE: Your carrier MUST support the Apple Watch. This is a list of Apple Watch supporting carriers

hhttps://www.apple.com/watch/cellular/

NO. The cellular radio has to be included during manufacturing. The GPS only Apple Watch (no Red Ring) does not include the cellular radio chip.

If you are within the 14 day return period, return the GPS only Apple Watch, and purchase one that does have a cellular radio:

https://www.apple.com/shop/help/returns_refund

If you are past the return period, then sell the Apple Watch and get a cellular model.

NOTE: The GPS only Apple Watch will do Everything the cellular model will do AS LONG AS your iPhone is nearby, where nearby is somewhere in the same moderate sized wood framed home.

https://support.apple.com/en-us/HT205547