Sadly the ability to show lyrics on musical files isn't as "standardized" as on video files. The MKV container is great for that, with the ability to add subtitle files as additional data streams, and most players are able to handle it. I wish the FLAC format would actually decide on an actual tagging standard for synced and unsynced lyrics data.

It's probably because the use of lyrics file is too much of a niche compared to video subs.