Text that is too small takes more time to read. Users may have to lean towards the screen, hold mobile devices closer, squint, or just concentrate more. As designers and developers, we strive to not ask for such extra effort from people who use or read our work.
On average, online text got bigger — at least in nominal pixel sizes — in the late 2000s and early 2010s. The exact reasons why are anyone’s guess, so here’s mine: