Because you don't own it, you licence it. That's the whole issue with digital distribution.
https://www.apple.com/uk/legal/internet-services/itunes/uk/t...
"""
- Purchased Content will generally remain available for you to download, redownload, or otherwise access from Apple. Though it is unlikely, subsequent to your purchase, Content may be removed from the Services and become unavailable for further download or access from Apple (for instance, because Apple loses its right from the Content provider to make it available). To ensure your ability to continue enjoying Content, we encourage you to download all purchased Content to a device in your possession and to back it up.
"""