Tags are objective to credit contributors to a map/song, list artists, lists genres, and that's it.
Lyrics have nothing to do with identifying a song on osu! even if they're well known because sometimes songs will share lyrics. It's literally like putting "Tatoe" in your mapset tags because it says it in the song but then you get maps with it in the title in your search. Essentially, that makes putting lyrics in the tags useless.
This is similar to the debate whether "deathstream" and "stream map" should be put in tags for players to identify a type of map they're looking for. Most would agree it's stupid because it has nothing to do with the song. The tags are meant to identify the song. For a system like the lyrics to work, it would have to have it's own database for people to search songs by lyrics. Completely defeats the purpose.