They have a couple of open source products. However they are not one of the big 4 as you have listed. I have heard MS employees talk about open sourcing some

They have a couple of open source products. However they are not one of the big 4 as you have listed. I have heard MS employees talk about open sourcing some of their bigger products and the conversation always comes down to the issue of the third party code (libraries, etc) they use that cannot be open sourced along with the main product. I'm not defending Microsoft in any way just giving an explanation I have heard over the last few years. I believe Mark Russinovich has talked about open sourcing things a number of times if you want to look into it further.