Piracy is relatively harmless to those who are selling tons of something. In fact it may help them due to increased virality. It could be thought of as deferred revenue or "unintentional marketing spend" if you've got something viral that piracy is helping spread.
It can be extremely harmful to small creators and smaller/startup companies who are attempting to make a living or get a business off the ground.
It sort of boils down to the diminishing marginal value of money. A small creator losing 20% of barely enough sales to make a living could find themselves in real financial hardship, while a large creator or corporation losing 20% of sales of something high-margin like software or media may not even notice.
Edit: Then there's perverse incentives...
Piracy is a driver for so much software becoming a "service" with components (or even the whole app) that live only in the cloud. The cloud is the ultimate DRM; you can't pirate it if you don't have it at all. This is creating a generation of software that can't be downloaded or run independently at all. I can run old DOS apps in a VM, but when this stuff goes obsolete or fails it will vanish forever and often take your data with it. It's also a privacy nightmare.
Piracy can also create a dumbing down effect if it requires skill. It results in older and/or more educated people pirating and therefore reducing the revenue outcome for the more sophisticated and higher-brow content they consume. I've wondered if this might not be a hidden reason behind the horrid state of pop music. Cheap catchy low-brow pop aims at a younger and less educated audience that doesn't know how to pirate.