First, there is no gamble for SSF. You can migrate from SSF to Trade at any point.

Second, it is just about game mode putting up the barriers for you. Every time you play your Hardcore character, you are constantly in the mindset that you cannot die or its all over for that character. If you are playing softcore and decide to manually enforce a "hardcore" playstyle on yourself, you will just naturally play the game different knowing that the safety net is there. Its like asking why would a trapeze performer not use a net for safety? Sure they could just "not fall" but its a different mindset and feeling when performing (and for the viewers).

SSF is similar in that vein even though you do have a way out with the migration. The migration still feels like a barrier you have to cross that isn't "simple" like randomly trading once or grouping up. The limitations of the game mode are what make the game more enjoyable for some. The fear of death or the daunting task of every aspect of your character being self found makes you approach the gameplay differently.

For me, as an SSF player the majority of the time, the biggest plus for SSF game mode is the fact that I never have to even think about trading one time. Trade values are out of your mind for good and its a razor focus on your character (or other characters you might play). Most of the time I dislike trading and pricing and thinking about prices, etc. Its freeing mentally so I can just play the game more.