How come I can remove my cat from a picture on my iPhone but am unable to do the same thing on my ipad

Presumably you refer to the new iPS/iPadOS16 feature for automatically clipping a subject or object from an image. 

Along with some other new elements of iPadOS functionality, this feature is not available for all models of iPad that support iPadOS16 - as specific system hardware (CPU/SoC and RAM) capabilities are required.

This specific feature is only available for these models of iPad:

• iPad mini (5th generation and later)
• iPad (9th generation and later)
• iPad Air (3rd generation and later)
• iPad Pro 11-inch (all generations)
• iPad Pro 12.9-inch (3rd generation and later)

More information about this and other features can be found within the iPad User Guide - this being a direct link to the relevant page for this feature:

Lift a subject from the photo background on iPad - Apple Support

Without knowing what year and model iPad you have/own,

Possibly because your iPad is simoly too old to support either iPadOS 16 or is too old to support the new iPadOS 16, Photos app background erase feature.

The new iPadOS 16 background erase feature is only supported on certain newer iPad Pro or iPad Air models and is NOT support on any new or older base model iPads or other older iPad models.

Sorry.