What makes All-Season tires for all seasons?

A 38% void to tread ratio.

"All-season" is a marketing term first coined by Goodyear a bit over 30 years ago and has been misleading people into thinking that they can get one tire that works great in everything ever since. Really what you get with an all-season tire is a comprimise that doesn't do anything particularly well.

This is why on both of my vehicles I have 2 sets of tires - a summer set and a winter set.

Mostly tread rubber mixture and its design, both being a more-or-less successful compromise for all weather situations and road conditions, including ice and snow.
It is reasonable to expect that such tires will wear sooner and won't excel in any particular weather conditions, but in snow and ice they will perform better then summer tires. Having all-rounders installed, however, makes you save some money and effort in that you don't have to replace them twice a year and stock the remaining set somewhere which, as a minimum, requires some space.
Note that in some legislations (especially in areas with heavy snowfall) the minimum thread thickness for snow tires is about double that for the summer ones. So your well-worn all-seasons may not be good enough if tested by police in wintertime or before entering a critical road section.

all season tires have larger tread blocks with more space between blocks to grip snow. They also have more sipes (channels to move water out to the edges). So they have more traction in snow and move more water than summer tires. But the larger gaps between tread blocks make more noise.

They're better than summer tires in snow, but no where near as good as snow tires. They're an ok compromise between the two.