There's some pretty good music done today as well, comparable to the old, but it's not necessarily mainstream anymore so you don't hear it as much on the radio. There's more and more focus on appearance, image and other extramusical qualities. The field is also a bit more spread out and the money is concentrating more on individual artists (think American Idol etc.), rather than groups. That's perhaps why you'd find less and less new "stadium-caliber" bands touring than just 20-30 years ago.
Having said that, I think majority of people are biased towards the music of their youth. If you grew up in the 70s, that's your thing etc. and most of the music coming since then is inferior.
I personally do think that for example Ed Sheeran or Twenty One Pilots are making innovative and good music, so there are some good ones still surfacing.
edit: For what it's worth, Alberto, I agree that I can't find as much good new music when compared to how many great songs I've found from 1960s to 1990s. I'm almost 40 now as well so I wasn't even alive when most of the (excellent) 60s-70s music was done. I discovered the songs years later, in my late teens/early twenties.
Because Music is made to be newer, younger so it never gets old like us
That make you feel strange with today's music
The difference between music today and music, say, twenty or thirty years ago, is that music today is incredibly easy to make. Anyone with a computer can be a music producer. So there's a lot of awful music coming out that would've never been picked up or produced thirty years ago, and even just the existence of the technology we have today has made people lazy and now there's a lot of generic music that all sounds the same. On the bright side, however, today there's a lot of great music coming out without needing to rely on the chance or luck of being picked up by an executive in the music industry. You can go on websites such as SoundCloud, and yes you'll find a lot of awful music, but you'll also find gems. The key is to explore music and look for what you want. Unfortunately, people nowadays don't want to explore music. Streaming services such as Spotify have made people expect to be served music. When I was a kid, I used to spend hours in record stores looking for CDs, and then I'd go home and put my newest purchase in my stereo system and lie down and just listen to the music. I still do that sometimes, and sometimes I can also spend hours and hours looking for new music online.
To be honest, whenever someone says they don't think there's any good music these days, my first thought is that they need to turn off the radio and their streaming services and look for music themselves. Some people think it's weird that I still buy CDs and also buy digital music rather than just stream it, but it makes me look for music more actively and thus makes me appreciate the music that I listen to a lot more.