Didn't say scale is hard, just costs.
Cdns are not free, they will pay for the egress bandwidth. Commercial DRMs normally function on a rotating key. Those keys are not free either and are charged during playback on clients.
The transcoding, storage and encryption at rest are once offs.
There are cheaper forms of drm but most would want the likes of widevine and fairplay. Players don't care, they will leverage the key request provided in a manifest but the frequency and production, often per segment, is the costly bit.
Often when you hear ifvstreaming vendors having concurrent view issues, it's these two components that fail, not the actual playback or serving of the video:
1. DRM key servers
2. Registration/user recognition.