Why does my cat scratch outside the litter box?

Usually because the litter is too shallow, too coarse or unpleasant underfoot — cats dig to bury, and one that cannot bury will perform the motion beside the box instead.

It looks like a habit and it is normally a substrate problem.

Depth first

Too little litter and there is nothing to dig into. Fill to the line the manufacturer marks rather than to what looks tidy.

Then texture

Coarse granules and crystals feel very different underfoot from fine clumping clay, and a cat that dislikes the surface completes the ritual on the floor instead. Switching to a finer litter often stops it in a day.

Then the box

A drum with a small footprint gives a cat less room to turn and dig than a tray did. Open, wide machines are easier for a cat that likes to excavate.

What it is not

Spite. And if the scratching comes with going outside the box, or straining, or a change in frequency, that is a reason to call a vet rather than to buy another bag of litter.