Why does all new pop music sound the same?

Because it has little or nothing to do with music and everything to do with current "lifestyles " .

That is an excellent 20min synopsis of this Horror Story:
"The Song Machine: Inside the Hit Factory"
I could not finish the book (too disturbing for me) but it finally "flipped" me onto "classical" for 90+% of my listening. I still follow a few "new" acts like Steven Wilson, David Sylvian, Bjork but the only recent 'top 100" album in my collection is "Black Star". Makes me feel old...

As if the music you listened to when you were a teenager didn't sound that way to your grandparents.

I’m with @onhwy61 ...you could say that all genres of music sound the same, although, If you are a fan and follow certain genres you’d be able to pick out what makes it different. 

What I have heard the last 20 yrs does sound more similar to me than not. Which most all of that music is not my cup of joe to begin with. The you tube video seems to make sense.
People relate to things they are familiar with. In the past few decades people have probably been raised without a connection to original music, acoustic instrumentation, creative music, personal ability to play an instrument even simple ones. So maybe that is why they want to hear the same not very good thing over and over. Just my opinion.