Social media looks busy because everyone can see it happening. Comments, shares and follower counts are all public, which makes activity easy to mistake for progress.
Email happens in private. People open, click, reply, book and buy without producing a single visible signal. Nobody congratulates you on a good send.
That makes it easy to underestimate, particularly in a business that judges marketing by how much of it is visible rather than by what it earned. The channel with the least public evidence is often the one doing the most work.