I suppose it's theft in so far as you're deriving value from something that others labored for. I think that sticks in peoples throats. You can go to the library and do that already, gain knowledge, start a business or whatever. But the scale feels impersonal and monstrous in comparison.
The same point was made by many in the music industry about huge scale Internet music piracy vs people dubbing CDs onto mixtapes.
Everyone laughed at them and rolled their eyes or called them greedy even though we now know that mass piracy was probably a push to break the music industry and force them to accept bad deals (like paltry streaming revenue). At the very least it had that effect.
Piracy has always been a major part of the computer and Internet industries, and yes the companies themselves have historically been massive hypocrites about it. It’s okay when they pirate but not you or anyone else. It goes all the way back to early companies stealing code and UI designs from each other.