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.