What is the secret behind "always soft" ice cream?

The softness of ice cream is going to depend on a variety of factors:

Use of gums and other binding agents, amount of sugar, the amount of fat, and especially the amount of "overrun" (air) that is churned into it during the freezing process. Less expensive ice creams will usually have a softer "chewier" texture than premium ice creams due to more gums and a great amount of air being churned into it to increase volume.

More air = more volume = more yield for the same amount of ingredients used.

Sadly, I think the answer is less physics and more chemistry--and not the good kind. The ice creams you are likely describing have been barraged with food stabilizers, emulsifiers, and other gelling agents which have nothing to do with sugar, eggs, and cream, and everything to do with getting texture out of "milk base" or whatever dreaded concoction frozen-yogurts, low-fat ice creams, and over-processed brands employ.

Make sure your ice cream has very few ingredients, lest you accidentally eat iced-emulsified-dairy base instead.

@Michael is right on, that faster, lower-temp freezing takes for smaller ice crystals which are creamier. Just make sure you know the difference. For interest, check out iCream which uses liquid nitrogen on the spot and claims very soft results do to the low, quick freeze.

There seem to be two approaches:

-Get away with as little water content and as much (emulsified) fat, sugar (bonus for adding some of a non-crystallizing variety like corn or inverted syrup), salt and alcohol as you can- they all lower the effective freezing point of the mixture, though fat in itself can get rather hard if it is the wrong kind. Alcohol in particular is surprisingly effective (once tried using a good amount of 160 proof rum, it really took freezing down to -20°C overnight to not be soft-serve-ish. Tastes great BTW). Works just as well with non-dairy bases!

-Go with "chemistry, and not of the good kind" approaches as described below.