What the kill boards don’t show are the hundreds or even thousands of freighters that travel into and out of Jita each day with no problem. Keep your cargo value low and your ship tanked and you will have no problem other than perhaps a bumper trying to extort a ransom from you or trick you into a duel. A friend or alt in a webbing ship can dramatically speed you into warp and reduce even that small chance.
The ships that do get exploded overwhelmingly carried more value in cargo than the value of the sucide gank ships used to explode them, usually much more. All of that information is in the killmails you are looking at and you can do the math yourself. They happen because the victim made themselves a profitable target.
As for the DPS, the data is all there as well and it is just a simple math problem. You have 6 to 25 seconds or so as a criminal to shoot a target before CONCORD arrives and jams and explodes you depending on the security status of the system. The gankers figure out how many high DPS ships they will need to sacrifice to inflict more damage than the predicted EHP of the target and kill their target before CONCORD gets them. If the potential loot is worth it and they can bump-tackle the freighter, they then go for it.
There is much written about what you can do to keep your freighter safe, and it is possible to operate with near perfect safety with some precautions. But regardless, you can be in the wrong place at the wrong time so never just load three months of work and autopilot your freighter to a trade hub while you go make dinner. Be aware, never overload, scout the route and most importantly never move so much cargo that you can’t afford to lose it and will ragequit if your ship explodes.
This is Eve and ships can explode when you least expect it or want it, even in highsec.