> ...this is Microsoft we are talking about.

Exactly!

More seriously, it not just about building a compliant web browser engine, it is about tracking the dominant browser engine which introduces their own "standards" along the way too.

Microsoft could have easily continued to invest in maintaining their browser engine, but it would have to be comparable to what Google is doing, and yet they'd always be perceived as "behind" due to non-dominant position they have.

I'd say strategically they do not want to invest in tech that's not winning (in marketshare), so when they are not dominant, they'll instead adopt and extend (not just web browsers, look at WSL too).