I suppose they can't easily, since the OSM license change.
My understanding is they'd have to open a lot of the geo data that they integrate together into their maps (POI, public transit, etc.), which might not be feasible as I suspect most of what they integrate is commercial and not owned by them.
(someone could check, but while Apple Maps had been using OSM e.g. for Pakistan, did they ever switch to post-relicensing data anywhere, or do they just keep using 2012 data?)
OSM is more of an all or nothing, everything has to be shared back, so it's hard to be pragmatic and mix different sources some open and some proprietary.
(and not every company wants to run a full fledged geo operation, and those can do it at scale will often end up keeping their data proprietary)
You can however, put separate and distinct data layers on top of your map, such as icons showing specialists points of interest, routes, track logs, shaded areas, contours and the like, then Share-Alike does not apply to these elements as long as they do not interact with the map underneath.