Because IMO you experience additional (and unnecessary) engine wear compared to shutting it off. Manufacturers know this, and those owner's manuals that list the conditions that qualify as Severe Service, thus requiring more frequent oil changes, include vehicles subject to extended idling. Idling means less efficient combustion, thus more unburned fuel. That results in more combustion chamber deposits. The exhaust catalyst will clean up the tailpipe, but that unburned fuel can wash down the cylinder walls, even with EFI. Some vehicles will have an idle speed increase circuit just to avoid extended low speed idle.

I think that engines are happiest when they are run at varying speeds, at varying loads. I have had to leave a vehicle idling for extended periods in extremely cold climate conditions, so you can't always avoid it, and at normal temperatures occasional idling isn't a problem. But extended idling of an engine designed to run up to 6000 rpm is bad practice, IMO.

Apart from the accelerated engine wear, why waste the fuel and pollute more? My idle limit is about 1 minute when parked.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue