(I don't know if this headcanon works since I haven't seen the show in forever but) you know how sponges in real life don't really move? So maybe in the show, sponges have really weak legs and have to excerize a lot to make them usable for a long time
I actually think so! In show, spongebob says his people are sedentary. And there's the whole thing about him being weak. "Built like a sponge". Sponges seem to be known for their physical weakness.
Not to mention the whole episode spongebob musclepants. Where he's shown to be considerably physically weak and other sandy episodes like hybernation week, his limbs can bend out of shape when he's over exhausted, even fall off!
There's also the episode Gary's got legs where spongebob gets so used to Gary doing stuff for him, his limbs basically dry up and turn to dust.
So I think while he has to exercise to keep his limbs in use. He is somewhat physically weak and when pushed to his limit, his limbs can become nonfunctional or even fall off.
Hope this answers your question!