Because a GPU setup and undervolted and power limited for mining is still running 24/7. It rarely ever hits idle and very likely had significant constant load on the VRAM chips, was run at constant fan speed, constant power draw and temperatures for many months on end. A gaming GPU will hit higher temperatures for a few hours every night or two at most and will be powered off and or idle for the significant majority of the time.
Setting a power limit means nothing IF you are one of those typical miners who also set a large VRAM overlock. Your mining GPU chip may not have been at full utilisation for potentially months on end, but your VRAM was.
So while your typical miner may believe this “myth” that an undervolted and power limited mining GPU is “well looked after”. The rest of us just roll our eyes out loud and laugh at the utter ignorance of such a statement.
That’s not to say a mining GPU is a massive risk of failure, but it does have a higher risk of failure.