Because it works in a beneficial way for all parties involved. At the end, we’re a community of music and video makers growing together. All YouTube/Twitch creators get high-quality music for their content and in exchange they spread the word about the artist who created it. That allows them to grow in social, book more concerts and usually send us more songs for your pleasure.
Crediting properly is the only requirement to avoid getting a strike and the video removed or matched by Content ID. Adding the original video link is also beneficial for your audience, because we always offer that download link your viewers will be looking for, as well as more info on the artist they found.
See How does it work to get the full picture.