If you look at the source code of most open source software, the devs typically install by default to arbitrary directories. They either don't know about, or care about, operating system standards. When Linux distributions package that software, they choose to change how that software works to align with standards. Usually their own standards, but those can often align with distro-independent standards too. But as a packager, you expect that you'll be modifying paths and making patches.
So really it doesn't matter where a random dev decides to install their software to by default, since either A) it's proprietary and it's non-standard anyway, or B) it's up to us to package it the way we want it.
Not everyone is using a package manager for every install, and even then it's not a magic bullet. I'm a Mac user and I've no idea why devs insist on installing to places like /usr/local or putting configuration in a dot file in my home directory when there's a standard place to put those (~/Library/Application Support), among other well thought out directory structuring[1].
Further to this, the moment you want to install somewhere else or have a dependency elsewhere (e.g. /opt/anything) builds break because of hard coded paths and poor assumptions. To top it all, I've no idea why a package manager like Homebrew then contributes to all of this by acting like no of this matters because a Mac is just like Linux, apparently.
If they don't like Apple's way then why not XDG[2]? Reinventing the wheel badly is more than just hubris, it breaks things and it's annoying.
So really it doesn't matter where a random dev decides to install their software to by default, since either A) it's proprietary and it's non-standard anyway, or B) it's up to us to package it the way we want it.