It's not. There were plans to raise threshold from 1.6 to 4mm it for winter conditions, but (luckily) they have not been implemented.
Michelin lobbied heavily against that... there were also some tyre test, where worn down tyres were compared. At 2mm, a Michelin Alpin (I think it was an A5...) fared better than tyres from other well-respected brands with 4 or even 5mm.
If you have a well designed tyre, you need profile depth only in regards to aquaplaning. This is admittedly dangerous, but Aquaplaning usually does not hit you from "nothing". As a driver you have much more control over this phenomenon than a tyre ever can: just lift you right food and adjust speed to the conditions... If you insist o doing 160km/h in a downpour, yeah, you need profile dephth. All of it. Get over on the right line and behind a semi (speed limit 80km/h, in reality they always drive 89...), and aquaplaning becomes next to impossible...
But other than against aquaplaning, profile depth is overrated. I was once caught by an early winter on seven or eight year old and nearly bald Michelin Alpin (the very first generation with the y-sipes) - thy had less than 2mm of tread left. I had just bought the car and had immediately ordered new tyres, but the snow came one day before the appointment to have them fitted. A fist to two fists high of new snow, unplowed, in the hills near the Nürburgring. No drama at all... Sipes (and a well-aging compound) rock.