Because the market is eternal competition. If one does something that works others have to figure it out and nobody puts their ideas in open source.
How much drastic would things be if these corporations do open source it? I like to think that markets are fairly efficient so they are fighting tooth and nail for micro-percentage points which granted can be billions but usually what these companies really do is short of fraud at times which can be celebrated by finance (Jane Street frauding Indian investors)
My opinion is that they aren't worried about their competitors so much as the govt.'s patching the loopholes that they do because the only way they are a net sum positive game (in my opinion) is that they make money from the losses of the average person and that too in fraudulent manners at time.
Jane Street's $5 Billion Derivatives Scam Rocks SEBI :https://frontline.thehindu.com/columns/jane-street-sebi-scan...