Then you’d be running unsigned, unreviewed binaries bypassing iOS controls directly on the CPU, which would be a security nightmare for iOS. This way it’s a completely emulated environment that’s isolated from the underlying system.
Running x86 binaries under software emulation, and forking off arbirary ARM binaries, aren’t the only two alternatives. You can do what Wine or gVisor does: run native-architecture code, but virtualize system calls as calls into the host binary.
You can't run arbitrary unsigned machine code on iOS. Interpreting it is the only solution.
Unfortunately, that slows things down with iSH a lot. iSH gives surprisingly complete Linux environment, complete with ability to install most of the packages you'd want. There's even an iSH folder in the Files app that gives easy ability to interchange files between iSH and the regular iPad environment. However, the emulation layer slows the Linux applications down quite a lot. Depending on your needs, though, might still be good solution for some things.
Millions of people use Android devices that allow sideloading, and it does not end up a "security nightmare".
Oh come on, security and privacy on Android barely exist at all except at the most general level. We're past the days when Samsung disabled all OS security controls on their flagship phone just to get the camera working for launch day, but Play Store apps are so infested with spyware and trackers the security of sideloaded apps is barely relevant.