There's not really a standard anymore for UPSERT to be added to. SQL has fragmented and since the last few iterations of the standard allowed for "additional non standard features", adding an UPSERT is perfectly compliant with the SQL standards.
And it's very useful in a ton of situations which is why so many other SQL dialects have added it. It's not like SQL Server is completely standards compliant so I'd rather have the functionality then have to use MERGE because it's "standard".
Microsoft doesn't need to make SQL Server look like MySQL. Quite the other way around.