In a recent post on the fascinating phenomena of composite citations in the New Testament, the host of the podcast remarked that the New Testament authors were comfortable using composite citations which indicated they knew their audiences were very familiar with what we now call the Old Testament. When you take a look at Romans 3:10-18 which was discussed in some detail during the podcast, it is one complex catena of 6 different citations combined into one composite citation.
Today, Christians struggle greatly with single Old Testament quotations that are longer than a line but the early Church seemed to be quite conversant with composite citations. Back then no one had ready access to the Bible as they knew it. There were even no public libraries that you could freely visit to reference these things. Moreover, the Roman church Paul was writing to were mostly Gentile so most of them had not grown up hearing and learning the Scriptures unlike Jewish communities.