With formats that came later, CD-recording remained expensive until personal computers were becoming widespread, and while CDs could be ripped or mp3s could be remixed on computers, you needed software that was often commercial and rarely included.
Digital audio then de-emphasized removable storage, re-introducing friction between exchanging music and listening to it on a portable player. Nowadays, most music exchange happens online, but the appeal of exchanging music in person endures even for generations that grew up with the Internet.
Cassettes had excellent usability for sharing: ease of duplication, ease of remixing, more rugged than vinyl, easier to hold than CDs, and maintaining its playback position at rest [1]. Couple that with authentic nostalgia, or yearning for an aesthetic and mood of times past. While the vinyl revival is centered around the deliberate experience of browsing, playback, and large-size album art, like enjoying an artisanal product made by skilled craftsmen, cassette revival celebrates the versatility of the format, its lack of restrictions, the opportunities for creativity, and its physicality on a very human scale.