Because they actually wanted to upgrade at QB. https://twitter.com/bkravitz/status/1246078579982696448

Here's a look at 2019... ¯\\_(ツ)_/¯

And in previous seasons: in both 2017 and 2018 BOTH Rivers and Brady were among the very best in the league.

Rivers 2nd in DYAR in 2017 & 3rd in 2018.
Brady 1st in DYAR in 2017 & 8th in 2018.

Rivers 4th in DVOA in 2017 & 3rd in 2018.
Brady 2nd in DVOA in 2017 & 7th in 2018.