Anecdotally, from when I worked there in ~2015, there were a few good reasons:

(a) a lot of Amazon's software was so old that it connected directly to the databases. We had to finish all of the SOA migrations that were long overdo -- all the misc random tools no one had gotten around to migrating -- before we could even contemplate moving off Oracle.

(b) it wasn't done as a simple port; instead, the systems were re-architected in the process to no longer share backing databases as at all, which basically meant it needed a complete rewrite, which took a long time to get funded and then a long time to deliver.