That struck me as odd too (I'm not really up to date on php7, and haven't considered php on windows in at least a decade). But perhaps it's because the PDO-driver is built on the old TSQL-interface, and MS own drivers haven't been ported to php7 yet (not with a stable release, anyway) ?

http://php.net/manual/en/ref.pdo-dblib.php

https://msdn.microsoft.com/en-us/sqlserver/ff657782.aspx

https://github.com/Azure/msphpsql/tree/PHP-7.0

I've used and liked PDO myself in the past. The issue is that even though the application would pretty much simply run on PHP7 (since it runs on 5.6 already) if MS had a compatible driver instead of contemplating a re-write