I believe the AVR oscillators run in RESET, and the internal RESET remains active for a predetermined interval after the external sources have gone inactive. This is accomplished by counting oscillator cycles.
There are prescribed initial values for many of the special function registers. It is also true that some bits in some registers are undefined. From the ATmega128 datasheet.
https://www.electro-tech-online.com/custompdfs/2009/06/doc2467-1.pdf
I think even your precious PIC processors have defined reset states for many registers in the register file. I know of no processor that does not set some number of internal registers to known states on exit from RESET. I think you are just wrong on both points for some processors. You may be correct about the oscillator being stopped on some processors, but any processor that can take an external oscillator probably isn't one of them.