Seems hilarious if you're like most people, install them, forget about them until they start leaking or stop working. Then yeah 20 years on a hot water heater would be an impossible number to you.
It's not impossible if you know there's a sacrifical anode inside them that needs to be replaced.
Typically the anode is completely gone within 5 years and the hot water heater rusts out in 2 or 3 years after the anode is gone.
How do you think steel ships last 50 years in salt water, same technology, sacrifical anodes.