Someone ties up $2,500,000 to short 1,000,000 shares at $0.0003/share.
The maximum profit is $200 (assuming he sells at $0.0001).
What if instead that person puts that $2.5m into a high interest savings account collecting 2%. They will make $4,170/mo.
No one with $2,500,000 (or more) to throw around trying to short some unknown penny stock is stupid enough to gamble on winning $200 at some unknown time when they can collect 20x as much, as a sure thing in interest.