I don't get why big companys are afraid of open source software

I don't get why big companys are afraid of open source software.

I know that monetizing open source is hard but in exchange they would have 8 billion programmers ready, for free!

Even if they do like redhat , as controversial as it is right now, they would be better off than just closing the source.

I would be willing to pay to have the license to modify my own software even if I couldn't redistribute it afterwards.