Artists don’t use free beats for many reasons — they’re heavily tagged, poor quality, etc. And for producers, it’s tough to find a talented artist that they want to do work with — maybe one rapper in one hundred is truly worth their time. Everybody and they’re grandma are rapping now, and to put it quite bluntly, most aren’t that good.
There aren’t many good resources for free beats. Beats posted on SoundClick say they’re “free downloads” but they’re full of annoying producer tags that detract from the song. Same thing on Soundcloud. A Google search sometimes turns up the odd site worth checking out, but most times the music just isn’t professional quality.
Producers usually charge $10-30 USD for a leasing license. The artist gets a high quality, untagged audio file for their project and (if the producer is about their business) a contract outlining the terms of the license. Honestly, $10-30 isn’t THAT much when it comes to releasing a song. There are other places you can “cheap out” and cut corners.