Vendoring used to be more common, even in Debian. But then there was an important zlib vulnerability (which IIRC could be exploited through manipulated compressed data), and they had to chase and fix all the many copies of zlib embedded all over the place. To make things worse, some of them were not only old versions of zlib, but also had modified that zlib code, so each had to be reviewed before applying the fix.

Debian (and other traditional distributions) learned from that incident, and started a strong push to not only remove vendored copies of libraries, but also when possible use a single version of these libraries. Which means that, whenever an important security issue is once again found in a widely used library, only a single copy has to be fixed and updated.