Of course, I can. There's the rub: I have seen reset pins, where the reset output is no true low ohmic output, but showed some strange medium ohmic behaviour, just in order to prevent it from being damaged by pulling it down outside, I guess. As if such a series resistor you mentioned is already built-in. Or maybe the reason is totally different?

Any such additional impedance can cause serious trouble, for instance, if you take a reset chip using an open drain output, just by the effect of voltage dividing. Other circuitry connected to the reset line can then suffer from unvalid reset signals and unpredicted behaviour.
But even reset chips with push-pull outputs can become very high impedant at the output, when the supply voltage goes down, so that the reset signal might become invalid at lower supply voltages.

Kai