Because emulators have to emulate the hardware through software, that and PCs use Intel or AMD CPU language, PS2 uses MIPS, Gamecube uses PowerPC, GBA uses ARM, etc. PCs don't understand the language or architecture that consoles use, so it has to trick or translate one language to another in order for the PC to understand. Top that off with the fact the processors, hardware, etc have to be synchronized and have to be emulated in unison as well. Emulation is imitating the console's hardware using software, this is why CPU requirements are high for emulators.
Intel x86-64 doesn't understand IBM PowerPC by default, the emulator translates the code for the PC to understand, but it has to emulate it; emulators rely heavily on the CPU.