Glitching attacks are typically performed by switching the supply voltage at quite high frequencies, a typical low-voltage detection won't trigger a reset under such conditions. And this is also why glitching attacks are often performed by spiking higher voltages, not lower. See for example Joe Grant's latest video on breaking crypto wallets [0].

Low-voltage detection is usually implemented as simple comparator which should trigger instantly, but often only on a single Vcc pin, and due to the decoupling caps found on a typical circuit design there is effectively an RC circuit that filters short fluctuations of supply voltage. So most low-voltage detection implementations only trigger on 'longer' periods of low voltage.

Traditionally low-voltage detection features (like brown-out detection) are there to guarantee functionality of the uC itself or the device the uC controls. It is typically not intended as a defence measure against these types of attacks. In fact, 15 years ago it may not have been much of a concern.

[0] https://www.youtube.com/watch?v=MhJoJRqJ0Wc