Had a group that would play DnD 3.5, where you need to roll to confirm crits (20 auto hits, roll again against AC to crit). We ended up rolling to confirm fumbles as well because catastrophic failure doesn't just happen 5% of the time. Imagine 5% of your army accidentally chopping their foot off or beheading their nearest kinsman every few seconds.
That's a 0.25% chance. Seems too low. I'd just repeat the test and if it results in a failure, it's a critical. That way the difficulty of the test would factor in.
Differing goals. For realism, you're right. Many systems forego a critical failure entirely because of that.
But fumbles are fun and dramatic. So while 1 out of 20 is excessive, having that danger lurking in every roll can be exciting. Of course you don't want to chop off limbs at every fumble, but chucking a weapon, breaking a bow string, insulting an official... They move the narrative forward in interesting ways.
More that KDM does the job for dealing with the idea of "stuff goes wrong". The games mechanics are built around it. Gear is important, characters are not. Which means stuff can go really bad very fast.
I will not discuss the merits of DnD as that will derail the thread and is rarely fruitful. Let's just say we don't have to agree on that to enjoy and celebrate our hobby.