They do organize, PhDs at my uni (grad students in general) have their own union which negotiates contracts, benefits and minimum wages. The issue is that there

They do organize, PhDs at my uni (grad students in general) have their own union which negotiates contracts, benefits and minimum wages. The issue is that there's only so much they can do given their position as people more dependent on the system than the system is on them. Especially when adding in how different PhD programs and funding can be between departments or even within deparments.