What it gains is the ability to include some proprietary bits, including their branding, the extension system, and the option to send telemetry back.

Just the same reason that the chrome binary isn't open source. Though MS doesn't make it's money on personal information, so the telemetry part is much more restricted.

>telemetry part is much more restricted

Are you saying that Microsoft is collecting less information than Google would in the same situation? I find that very hard to believe looking at Microsoft's history. Do you have anything to back it up? The only comparison I can come up with is Windows 10 versus ChromeOS.

Here's google's quarterly 10-Q statement.[1]

"We generate revenues primarily by delivering relevant, cost-effective online advertising." For the quarter ending June 2018, google advertising revenues were $28,087 million, out of $32,657 million total. That's 86% of their total revenue, for those keeping score at home.

Here's the relevant section of Microsoft's financial statement for the same time period. Advertising revenue is $6,100 million, out of $36,400 million. That's 17%. Microsoft doesn't even include the figure as a sub-line item, because their money makers are Azure and Office 365.

I understand that MS has a history of being the great opponent to software freedom. I understand that they've made moves that are anti-developer, and anti-customer in some ways. But their business model has never been based on collecting information. It was based on selling licenses, and now it's based on paid PaaS and SaaS.

With that in mind: it's hard to compare giant, decades-old corporations like this. We will inevitably come up with an incomplete list of anti-privacy things that both have done. But the profit motive lines up pretty clearly here.

But because I can't resist... google privacy issues have a wikipedia page: https://en.wikipedia.org/wiki/Privacy_concerns_regarding_Goo... . There is no analogous page about Microsoft privacy issues.

[1] https://www.sec.gov/Archives/edgar/data/1652044/000165204418...

[2] https://www.microsoft.com/en-us/Investor/earnings/FY-2018-Q4...