Since many brokers disallow investors from shorting sub-$5 stocks, why don't all companies split their stock until it is sub-$5
A stock split can force short sellers of penny stocks to cover their shorts and cauuse the price to appreciate.  Example:  Someone shorts a worthless pump and dump stock, 10,000 shares at .50.  They have to put up $25,000.00 in margin ($2.50 per share for stocks under $2.50).  The company announces a 3 to 1 split.  Now the short investor must come up with $50,000.00 additional margin or be be "bought in". The short squeeze is on.