It is samey, because everybody does the same thing. You can’t really differentiate music by different performers anymore. Each song is the same.
There used to be a cost to producing music; you needed a studio, producer, physical copies of the samples you wanted to use (in case of most rap), etc. Now you can just boot your computer and use one of the many digital studios.
It is a blessing in some ways, but a curse in most I think. Quality is by no means the element by which (popular) music is judged. It is all about presentation, which somehow means the most deplorable people get the most praise.
Note: I am talking about what is mainstream and played on still high quality radio. I don’t listen to rap stations and luckily there are stations that have a good selection of older music and only play some of the new stuff. The new stuff there is already worse than music from the past though.
If you look at the rap and hip-hop scene however, the quality has plummeted even more. It is just one homogeneous soup of crappy, lazy music. In no way like the music of 2Pac, Notorious B.I.G and Outkast but still all the same as the other new crap.