Games. Seriously. You have a lot of old (even new) games that depend on 32-bit binaries/libraries. And that’s why the recent “clash” between Canonical’s Ubuntu and Valve’s Steam[1].
Honestly I can’t recall which gen dropped it, but presumably the moment they reached their EoL for the last non-64 bit device they had no reason to maintain the development cost of having a 32bit OS, maintaining support for communication between 32 and 64bit apps, nor the cost to users (doubling the size of the resident system memory, confusion of some apps running on some device but not others, etc).
And then knowing that if you drop 32bit then you can gain space on your silicon.
What irks me is that devs saw the first 64bit devices come out, and didn’t go “I should recompile my code”. Honestly the fact there are so many new apps made for pc that aren’t 64bit is mind blowing to me.
Each and every version of macOS for the past 10 years has shipped with 4+ years old versions of OpenGL.
For example El Capitan (the last version without Metal) was released in 2015 and shipped with OpenGL 4.1 which was released in 2010.