For essentially the same reason that Rosetta 2 can't be used to run Windows - they're userspace JIT translators, not system-level emulators. The Classic Environment was, for all intents and purposes, an entire virtualized PowerPC system running Mac OS 9, and allowed software running inside it to do some pretty wild things like patching system "trap" routines, writing graphics directly to the framebuffer, or even setting breakpoints in and handling exceptions from other applications.
For essentially the same reason that Rosetta 2 can't be used to run Windows - they're userspace JIT translators, not system-level emulators. The Classic Environment was, for all intents and purposes, an entire virtualized PowerPC system running Mac OS 9, and allowed software running inside it to do some pretty wild things like patching system "trap" routines, writing graphics directly to the framebuffer, or even setting breakpoints in and handling exceptions from other applications.
The Classic environment depended on the system having a PowerPC CPU.