Because Office 2013 no longer receives security patches, for everyday work. It’s suitable for air-gapped machines, virtual machines for legacy software testing, or offline document editing only.
If you bought a physical DVD box, check the card inside the case. If you bought it online digitally, search your email inbox for the "Microsoft Order Confirmation." If you cannot find the key and it is not in your Microsoft account, you will unfortunately not be able to activate the software legally.
Additionally, downloading Office installers from third-party "freeware" or "torrent" sites is highly discouraged, as these files are frequently infected with malware.