Initial state

In the IC's documentation, there is a caution to the effect that the initial state at power-on is undefined, but why is this so?

In logic ICs where the logic is determined solely based on pin inputs, this is never the case.
If registers, counters, SRAM, etc., are incorporated in the IC, the equivalent output of their each bit is configured as shown in the following figure, and the characteristics (Q = 1 or 0) at power-on differ depending on the manufacturing conditions and operation conditions (voltage, temperature, etc.).

For a bit whose state after reset is defined, the Q is logically set to the prescribed state through the reset operation.