Usually the reason that big companies open source big longstanding projects is to commoditize a complement of their main business:
- Search makes billions of dollars, and you need a web browser to search, so Google should make a fast and secure web browser (not to mention you can fill it with all sorts of dark patterns!)
- Users purchase Windows to run applications like spreadsheets. We should offer a free high quality IDE so that more applications can be made, increasing the value of the ecosystem.
- Java: Sun sells hardware, and software is the complement to hardware, so we should make a new programming language (???). To be honest, this one always seemed suspect, and maybe that explains why Sun went out of business :-/
https://www.gwern.net/Complement
https://www.joelonsoftware.com/2002/06/12/strategy-letter-v/