The answer is quite simple. Ronnie's dad liked to see people (and Muppets) sweat.
It all stemed from Ronnie's past as a young wannabe producer, much like his son. Ronnie was hired by a Muppet to act as a gofer. Let's face facts: a human working for a Muppet is going to be chaotic. Young Bernard (or "Bernie", as he was affectionately called) was asked to routinely perform tasks that aren't humanly possible. These included changing the sets in under five seconds and successfully performing in glorious closing numbers that were never rehearsed. Additionally, Bernie was not allowed to blink or show his legs. Bernie just could not keep up, and was quickly fired as a result.
Following a brief stint as a moving target during the War, Bernie sunk into a sea of depression. After six months of self therapy, Bernie figured that Muppets were the cause of his troubles. From then, Bernie continuously stalked different Muppets, hoping to get revenge. Some of his evil plans included sending a crony (disguised as a restaurant owner) to stalk and kill a Muppet, sending a second crony to rob a fashion designer and blame it on a Muppet, and disguising himself as a Muppet monster in order to eat a non-monster Muppet. All ideas failed miserably.
His chance finally came when a young Muppet arrived in New York to get his musical produced. Bernard, by then a well-known producer, decided to grab the show. As he told his cronies, "No one would be insane enough to produce something like this... except me! Maniacal laugh! Maniacal laugh!" Bernard planned to force the Muppets to get the show ready in two weeks, something that isn't humanly possible when it comes to Broadway. But after meeting with the show's writer, Bernard realized that he wasn't dealing with humans. At the last minute, Bernard paid a taxi driver to hit the show's writer with his cab. With their writer and boss missing, the Muppets spent the next two weeks sweating like pigs (some literally), much to Bernard's delight.
On the night of the premiere, Bernard had a big grin on his face, feeling he had finally gotten his revenge. Not only did he make Muppets suffer while putting a show together, but he also knew that making a good Broadway show in two weeks isn't humanly possible. As the curtain went up, Bernard once again realized he wasn't dealing with humans. Any production that a Muppet is in charge of is bound to be a success, no matter how rushed it is. In a last-ditch effort to get revenge on Muppets, Bernard and two of his cronies (an accountant and a delusional ex-Nazi soldier) attempted to blow up the theater. While Bernard did succeed in getting rid of the audience, the Muppets naturally survived the explosion unharmed. Frank Oz and Mel Brooks both found this story so hilarious that each used elements for their solo directing debuts. As for Bernard, being a non-Muppet meant that the explosion sent him away with the audience.
An alternate answer:
"If you're wondering how he eats and breathes and other science facts, just repeat to yourself 'it's just a show. I should really just relax'". - Joel Hodgson