Why can’t we use cellular data to update it

To update over the air with cellular data, you need a 5G capable iPhone, connected to a 5G network that supports that sort of data usage.

See -> Use 5G with your iPhone - Apple Support

“Data Mode

• Allow More Data on 5G: Enables higher data-usage features for apps and system tasks. These include higher-quality FaceTime, high-definition content on Apple TV, Apple Music songs and videos, iOS updates over cellular, and automatic iCloud backups.“

asamoah165 wrote:

Why do iOS have a date?

Because wifi is required to update over the air.