Because currently (Intel Macs) both the host and the guest are Intel: so, virtualization is enough (same architecture); while on an M1 Mac the host is ARM and thus you can only virtualize ARM: so, running an Intel OS (for example, Mac OS X 10.6) on an M1 host requires a full x86/x64 emulation.

Not sure if it's clear (maybe others can explain it better?)...

(For example, if you want to run an old PowerPC version of Mac OS (X) on an Intel Mac, you must emulate a PPC system (with QEMU); likewise, on an M1 Mac (ARM), if you want to run an Intel OS in a virtual machine, you must emulate it: it cannot be directly virtualized, as the processor architectures are different.)