Because Linux (and other nixes) have their root in multiuser/time-share systems/servers. Protecting the system* from the users was important, and protecting users from other users equally as important. Protecting the user's $HOME from themselves/user-level programs wasn't as much of a concern, the user was assumed to be responsible enough to manage it themselves.

Linux /home is far from a free for all. flatpak, landlock, selinux, podman, firejail, apparmor, and systemd sandboxing all exist and can and do apply additional restrictions under /home

>Why does every Linux distro under the sun try so hard to protect the garbage under /usr/bin/ and /etc

Because a compromised user could infect shared executables and spread the infection. A bit harder to do with etc but for sure possible. The main target would be infecting bash and you are done from the get go.

>when literally the only files that matter to me are in /home, which is a free-for-all?

The home folder's read write is usually restricted to the user. The only scenario where this isn't the case to my knowledge is Ubuntu where others can read it, but this is just a huge flaw in Ubuntu that almost no other distro has.

The first point is fairly obvious and the latter point is not true (AppArmor etc)