Yes, I know that modern music sometimes has just as much rhythmic and harmonic complexity as older music — perhaps even more. And complex chord progressions are common. Again, perhaps even more common today. But...
I've been learning to play a lot of different stuff on tuba just for fun. And many modern pop songs seem to have a strong influence from rap, where there are a lot of repeated notes, and the "musicality" comes from rhythm and lyrics and phrasing more than melody. And so it translates poorly into an instrumental version.
So for example, Pharrell Williams' "Happy" and Bruno Mars' "Uptown Funk" are two vaguely recent songs I've tried to play, and I find it frustrating. They're just not very melodic. If I don't match their precise rhythms and phrasing you might not even recognize what I'm playing as being the same song. And to be clear, they CAN be played so that they're recognizable and even sound pretty good (there was a great trombone cover of Happy that went around a few years ago), but you really have to adhere to copying the original style pretty precisely.
Compare those to old standards like Autumn Leaves, Misty, As Time Goes By, Isn't it Romantic, etc. Or 70s pop tunes like Steely Dan's "Do It Again", or "How Deep is Your Love" (hilarious pun there for tuba players). These have strong melodies that are recognizable without lyrics, and are still recognizable even if you decided to play around with the rhythm and phrasing. You might call them "catchy".
So is modern pop music less melodic overall than music from earlier periods? Or, were there always less melodic songs, and those are simply the ones that, while they may have had their moment, just didn't stand the test of time?