I’ve heard this comparison made time and time again. One thing I’ve come to realize is that it’s not just older folks who think that songs from the past sound ‘better,’ I’ve met millennials and teens who say they prefer listening to songs that their parents enjoyed when they were kids themselves.
Still, I believe there are many good songs being made today, maybe just as much as bad songs were made in the past. The difference I think is that too much of today’s music relies on a hit-making ‘formula,’ resulting in, as you pointed out, a lot of the songs sounding the same. On top of that, marketing and promotion help to push certain types of songs to the top, so they end up being played more often and get most of the attention.
Also, a lot of it is digitized. The cost of producing music using acoustic instruments became prohibitive at some point, and the emergence of ProTools and other music production technologies made it cheaper. However, cheaper hasn’t meant better, so while it’s easier for people to create music, a lot of the output is just too digital. It’s little wonder why some artists have been turning back to using live instruments upon finding ways to reduce cost.