The problem is due to a process known as hydrolysis where water gets into the sole and a chemical process causes the urethene to degrade.
The problem also seems to affect shoes which are stored for a while, perhaps put away damp, even brand new but stored can be affected. Frequent use appears to help, perhaps by pressure keeping the moisture out?
There are also manufacturing difference. PU is expensive and some manufacturers put talc into the mix to bulk it out. This weakens the PU and, I suspect, as talc is hydrophillic at low humidity this will make matters worse.
It would appear that manufactures of cheaper shoes don't expect you to get more than 4-5 years life whether you wear the shoes of not.

Mmm, I’ve had my Scarpa mountineering boots for 30 yrs and they are still going strong*

*PS added - Vibram rubber soles.