It’s probably because they’re singing about more than just wanting to hold your hand…
Tyler Cowen on the word count of pop songs:
Average word count of top-ten songs during the 1960s: 176
Average last year: 436
That is from Harper’s Index, August issue. I don’t think it can be a pure length of song effect.
I would also argue that the music itself has become more complex (different synthesizer modes and track sampling mean that there are more “instruments” to keep track of). We could be finding a musical Flynn effect much like we’ve seen in television.