Because it costs money to open source something as large as an OS, because you need to vet the source base to ensure that you own the rights to publish all of the source code under whichever license. To execs, it looks like throwing money into the toilet.
thats a strawman, here's a summary of ownership from google: Be Inc. => Palm => PalmSource => ACCESS Co.
this is ho many times they moved it around or sold it. each one of these times they had to do due diligence before same as you do with any ip acquisition. okay if you dont want to do full opensource, how about taking a few modules and just bsd-3 or MIT-ing them. it shows Intent, how many time any VC has done this in last 20 years? none that I recall. this is the problem, big do-gooder talk about changing the world, but at the end of day they are out to JUST make money.