Shitty detectors; people simply forget to check on them or don't bother to install one in the first place or don't even know they should have one; expired detectors that don't work anymore but also don't alert that they're non-functional. Plus, older people tend to get worse at maintaining out-of-the-way stuff like that, more forgetful, harder to get around, may visit entire sections of their house (like a basement) almost never so just forget about it or don't notice signs that something needs to be replaced or repaired.

Check on your parents' smoke and CO detectors next time you visit!