It's literally the same reason most companies give students essentially fire sale discounts (often just free) versions of their software.
They're probably pirating it because they aren't deriving much if any revenue from using it yet (it's either for personal use, they're learning a new tool, or they have no money). But having that person familiar with your software and trained on it means that if they reach a spot where they're using it profitably for business - it's basically a guaranteed sale.
"Sure thing, how many do you need?"
When I hesitantly reminded him we hadn't discussed prices yet, I was told it was free as they really wanted to snare us in as users before we wound up in paid work where we'd need to be productive employees fast.
I have since had three employers buy full licenses and support deals. Good call by Protel back then.
Instead, the value proposition to switching to paid became once it had established value to me, and I could benefit from the reduced exposure risk of running malware in the cracked version, and possibly get updates if they affected major things like stability, or new tool-sets or access to online data that was useful.
Very true. If they were never going to pay anyway, you haven't lost anything if they use a crack. It still irks though!