On top of not having source concerns, licensing makes sharding less useful. MSSQL on Linux exists but it comes with a bunch of caveats so most people will stick with Windows. Therefore, each instance of the server will MIGHT be Windows License (depending on various factors) + SQL Server licensing and that licensing is not cheap, therefore, with MSSQL, if at all possible, scaling up and not horizontally is generally preferred. You do have Availability groups if Write Primary + Read Only Secondaries work for you.

Also, MSSQL is clearly on maintenance mode. Microsoft continues to support it and sale it because $$$$ but it's not a focus.