As izzy pointed out it's because teams go absolutely crazy if left to their own devices.
And them they blame Pirelli when things go wrong...
Things i can remember, running tires on the wrong side, running way more camber than recommended, and running extreme low pressures for way longer than the tire was recommended for.
This last one, Spa 2015, led to a tire failure for Vettel and Ferrari instead of accepting the findings(pretty reasonable ones imo), "this type of tire was suppoused to last 20 laps and you did 35", "you were also running well below recommended pressures", made a huge fuzz... on the next race Pirelli started checking for minimum pressures.
So at least for Ferrari there's an element of "they brought this on themselves".
Still the pressures they recommend seem way too high for me... there's room for not playing that safe.