By producing their own browser, Microsoft can at least be assured that it works consistently under their own controls and can be used to power other features in the future for Windows. First feature is the Cortana integration in Edge. Good luck getting either Google or Firefox to agree for a deeper integration level.
You cannot integrate Cortana in Chrome like the way they did with Edge. Cortana is being used to collect information and provide better ads over time based on what customers are searching. It is the same market as Google is in and they're not going to do anything to lower their ads revenue and Microsoft is not going to pay them to bundle Chrome onto Windows, that's just not going to happen.
It is generally not a good idea to depend on a third party to provide the best browser (or any other types of tool) on your platform where they can control all of it and can deny whatever you want from them. Especially, when such parties are only doing it to drive business to their own solutions. Office vs Google Docs, Bing vs. Google, Cortana vs. Google Now, and so on.
Also, none of these things are being given away for free, they all have some costs for various groups. Companies who want to use VS professionally will pay tens of thousands of dollars per year for the support and maintenance from Microsoft, sometimes even more. Remember, a big part of MS revenues is from the enterprise market, not the regular consumer market.
Microsoft is not doing anything for free, that makes zero sense and would result into their entire management staff fired immediately. There are revenue models for each free solution from Microsoft, they're not in the market to produce free software, they're in it to make money for their shareholders.