Infringement != theft
And in any case you are arguing about semantics, "theft" is also technically not a singular legal definition but an amalgamation of different civil and criminal legal statues.
In most jurisdictions intangible property also does fall under the broader definition of theft: https://en.wikipedia.org/wiki/Theft
A key term is intent to deprive, hence most people nicking a car will be done for taking without consent - no need to prove an intent to permanently deprive.
Copyright infringement is a whole separate law and is clearly not theft as there's no intent to permanently deprive.