[0x0]

Probably because the 6502 CPU had instructions for optimized access to the zero page (first 256 bytes of RAM), this would also apply to C64 and NES etc. If you want to use "Indirect Indexed" memory accesses then I think it also has to go through an address held in the zero page.

See https://www.nesdev.org/obelisk-6502-guide/addressing.html

[pajko]

The AVR architecture puts its registers starting from address 0 of its data memory. As a twist, the program memory also starts at 0 (due to the Harvard architecture).