Why all characters' arms/legs in Adventure Time are so soft?

It is most likely artistic license based on the style of the show, which is sometimes called Noodle People:

An art style in which characters are drawn with exaggeratedly long, thin torsos and limbs.

As the Western Animation Examples section mentions:

• The general art style Adventure Time, though there are exceptions.
  • Jake takes it Up to Eleven. Being able to change shape at will, he often moves his limbs in ways that imply he has no bones whatsoever.

Which nearly all the humanoid characters adhere to:

It may also be a callback to the classic style of animation where characters always acted like this due to the limitations of animations, sometimes called Rubber Hose Limbs:

This is when a cartoon character moves without any visible elbows or knees, so his limbs aren't stiff, but rather, bendy, like rubber hoses.

In cartoons made before the mid to late 1930s' or so, this was intentional, for practical reasons rather than artistic ones. It all started with Felix the Cat animators Otto Messmer and Bill Nolan, and it was meant to prevent the motion of the limbs in question from looking like they were drawn through a strobe light and flickering — the basic idea was that if you didn't draw joints, you could make absolutely sure that the limbs in one frame overlapped with where the limbs were in the last frame.

These days, this is a deliberate artistic decision on the part of the animator — either to creep you out; to create a fun, zany, or cute character; or as a deliberate nod to the Golden Age Of Animation. In the early days, however, it was just how things were done.