It's that you "copy" it. That's the "right" that copyright law extends to creators.
Technically (and it's complicated these days because copyright law was mostly written well before computers and the internet existed) any time you create a copy of someone else's creative work, you need an explicitly granted right to do so. You can _read_ a book, but you cannot write down all or part of what you just read. You can listen to a song, but you cannot perform all or part of that song (An Australian band called Men At Work lost a copyright case for a flute solo that contained 2 bars/11 notes of a 1930's folk tune "Kookaburra" who's copyright had been sold/bought in 2000.)
There are some explicit legal exceptions to requiring a license, most commonly people talk about "fair use", but that's again a "pre internet" body of law, and is extremely open to interpretation as to how it applies to digital copies in computers.
Technically (and it's complicated these days because copyright law was mostly written well before computers and the internet existed) any time you create a copy of someone else's creative work, you need an explicitly granted right to do so. You can _read_ a book, but you cannot write down all or part of what you just read. You can listen to a song, but you cannot perform all or part of that song (An Australian band called Men At Work lost a copyright case for a flute solo that contained 2 bars/11 notes of a 1930's folk tune "Kookaburra" who's copyright had been sold/bought in 2000.)
There are some explicit legal exceptions to requiring a license, most commonly people talk about "fair use", but that's again a "pre internet" body of law, and is extremely open to interpretation as to how it applies to digital copies in computers.
https://www.copyright.gov/fair-use/more-info.html
https://fairuse.stanford.edu/overview/fair-use/what-is-fair-...