Some albums do not include any lyrics' sheet, but often the lyrics are officially available (for free) on artists' website. Does that means that words are considered less important than music, and in some ways superluous to be offer together with the music?
The obvious answer (when talking about recent releases) is that lyric sheets cost money to produce. If you can put them online for free, why waste money on something that isn't as important to the core of the release?
Other bands, like Tool, don't include lyric sheets because they don't necessarily want to spell everything out for the listener. They want to make you think about the music, or interpret whatever is being said in your own way.
It's because lyric sheets cost more money. Some bands like to put the lyrics of their songs inside the booklet for the CD, while some don't like to print their lyrics at all. It isn't like they're going 'it isn't important'. Also it frees up the space to put some cool artwork in the CD case, which (and this last bit is just my opinion) is a better addition than lyrics.