Why no official Apple weather app for the iPad?

Not all Apple Apps are available of both iPhone and iPad - some being limited to one or or other platform. The “official” Apple Weather, Calculator, Wallet and Translate Apps are all iPhone Only - as is the App Library function. As for Apple’s design and development decisions, this information is rarely shared outside of Apple itself - and Terms of Use that govern participation within these Support Communities does not permit is to engage in speculation.

Whilst the [iPhone] Weather App is not available for iPad, your iPad does have a native Weather Widget, accessible from the Today view of your first Home Screen, which links to the Weather.com website.

Apple do invite constructive feedback, comment and feature requests via their product feedback pages:

Feedback - iPad - Apple

For the same reasons Apple never created an official calculator app for iPad.

Apple could not come up with its own unique iPad design for either app.

Apple did supply a weather widget for iPad, in iPadOS, that comes in three different size and info variations.

Use widgets on your iPad - Apple Support

I use this version of the iPadOS weather widget

Best of Luck to You!

Why does it need to be an official app?

From the desktop, swipe right. You should see a weather widget there giving the current temp for your location. Tapping on it jumps you to The Weather Channel's web site for more details.

So am I. I check the local temp that way all the time. I did it just now when I posted.