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].
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.
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.
Assuming you mean Windows, there are still a nontrivial number of PCs in the wild that are running a 32-bit OS. It's not huge -- about 1.5% of gaming PCs according to the Steam survey, probably a bit higher for home and office computers -- but either way it's enough that building applications as 64-bit-only is problematic.
But more seriously: why ship 32-bit only software. 64bit has been supported by all cpus intel and amd have shipped for more than a decade now. I had a consumer 64bit Athlon in 2005/6 that I could afford on a student budget. Given the perpetually increasing cpu demands of modern games, that you'd throw away something in the order of 15-20% perf by not support 64bit just seems insane.
Seriously: make 64bit your primary target, and if that hurts 32bit perf, that's those user's problem: they chose not to buy a high perf system.
But why? The Cave was released in 2013. We're almost two decades into the 64bit transition.