Assuming you mean "non-keyframeable" - I've asked that question, too. It seems it's just a thing nobody has given consideration during development. If you think it's keyframable (which it is not) and wonder what use that would be, you should get into a mathematical mode of thinking. It's all about control. In any computer simulation forces are just multipliers that effect the motion of objects/ points. Basically the algorithms themselves don't care much if those multipliers change over time. With that in mind, you can use even Gravity to mimic other forces in the real world or make forces have no influence.