C DM's answer was the same as I going to say - weather requires a lookup to get the current weather.
I'm also betting there's some UX implications of having an icon not being more or less visually static. The clock icon is visually static to me - I've never noticed the clock icon's animation or it's face being chronologically correct.
In regards to the data usage and the constant updates even when you don't really want to see the weather, all I can say is that it's already happening on the Apple Watch and the temperature complication.
Probably battery drain concerns. It would be pulling weather info constantly to show it.
Then again it already does that to show it on Notification Center so I don't know.
At least with the NC, you need to actually open it before it updates. While the homescreen is likely to be shown much more often and with less likelihood that you're using it to check the weather.