Why does WP’s built-in thumbnail functionality suck so bad?

I’ll probably get flamed for this, but WordPress’ thumbnail functionality is the single largest pain point in trying to build a WP theme. It’s all well and good if you’re building a new theme from scratch, but if you’re coming in to build a new theme for pre-existing site? Heaven help you.

So this isn’t meant to be so much of a rage post, I seriously want to know why one of the most coveted carrots ever dangled in front of theme developers (WP’s thumbnail functionality) is such a PITA to work with and why is it so fickle? What’s the root cause, and what can I do to make it suck less bad?