Why wifi require when update appears in iphone 13

Because you cannot use cellular data to process the update — unless you use 5G cellular data.

Use 5G with your iPhone - Apple Support

Wi-Fi is not required if you have a strong, legitimate 5G cellular connection. Note, things like "5G E", are not really 5G.

Based on the context of your question, it appears that your phone does not have a 5G connection.