Lots of reasons here.

I find myself starting way more than I finish. I get distracted very easily.

I may put one aside because I decided it needs some aftermarket item that I don’t have.

want to be able to paint it a certain way and set it aside until I get a little more practice with the technique I have in mind.

Get burnt out with all the little fine small details or repetitive work.

Involves photo etch.

Burnt out on major cleanup work of parts.

The list could go on, LOL

But I really do admire the modeler that only builds one project at a time start to finish.

Plus it’s a pure joy when I get a model that I can’t put down until its done.