Adobe for a loong time (up until CS6) didn't give a fuck about piracy. Everyone with an interest in media when I was in school had a keygen and learned Photoshop, some even started small solo businesses with a pirated version until they had enough money to buy the actual thing.
On top of that, developing for photo, video and audio is hard due to all the maths involved. The amount of brains capable of that wizardry is finite, the amount of brains able to do open source work in that field is even less, and other FOSS projects compete heavily for these brains.