SD and microSD nominally inhabit the same space, but microSD is effectively limited to storage whereas SD can be used for other things; it's just more flexible to provide an SD slot, if you can spare the lateral space. Among other things, microSDs are so tiny that it is often more practical to put them in an SD adapter whenever you need to handle them. That's why SD adapters are so widespread, and you'll likely find one in the package whenever you buy a consumer microSD.