Satellite (you probably mean "aerial", commercially avaliable satellite imagery just doesn't have enough resolution) is perhaps map imagery, a supplementary source for maps; but not map data at all: maps are an interpretation of the world, imagery is "just" raw pictures.

For example, my city gets aerial imagery taken ~yearly, with resolution better than 1 ft/px. This is very precise, and the updates are frequent; yet, still insufficient to answer typical mapping questions, such as "can I get there from here, using this vehicle?" That's what a map should be able to answer: both map and aerial image can tell you "there is a vehicular track", but only the map will tell you "and it's private, and even if you don't care for signs, this gate over here is locked, so no, this does not route; you can't go over that road here, as it has a bollard in the middle; that bridge is too flimsy for you; that street is one-way but opposite; you need to go that way, but there's toll collected; otherwise, there's a lengthy detour."

Only some of these features can be inferred from an image. (Of course, the map needs to be kept up-to-date with all this data, but that's a very different story)

Satellite images don't work as maps simply because trees exist. I grew up in a national forest and looking at satellite images you'd never know my road existed, or all the roads that connect into it. The trees make a canopy over the road and it's impossible to tell there's anything under them.

Same problem with rivers... a lot of rivers show up the same color as the forest on satellite images so you'd never know there was a river there even if you can "see" the water.

Depending on the angle of the satellite, this problem exists in large cities as well. Roads are obscured by skyscrapers even at the slightest angle, and roads disappear from the picture without telling you if they went underground into tunnels or just ended.

Now, satellite pictures overlaid with map data, that works. But just satellite pictures by themselves isn't enough to navigate.

Satellite images are not map data in the same way that a photo of a machine is not a blueprint.

I think if the machine was two dimensional and the photo had a measurement scale then it would be as good as a blueprint.

The machine is not, and neither is the world. Even if it were, it would still not be as good. It would not tell you about materials, tolerances, or many other important things.

It isn't a requirement for a map to capture all characteristics of an area to be considered a map.

That does not mean you can just leave out anything you want, though.

No, many aspects are not captured by a picture. E.g. borders.

A map that doesn't capture all information about an area doesn't make it not a map.

I found OSM to be better than the competition for hiking (it has more trails mapped) and walking in cities (pedestrian only shortcuts are well mapped).

To me, map data means just the data required to know what is where. I wouldn't count "satellite" images, geocoding, street view or anything like that as map data, but their own, distinct things. As it stands, OSM is one of the big 3 in that field, together with Google and Microsoft. Both of the latter might be as good when it comes to streets and buildings, but OSM destroys them when you also include hiking/bike trails, naval marking, traffic lights, power lines, etc. - some of which the other two don't provide at all.

Not to mention that afaik the "map data" itself isn't actually available from either MS or Google - they only sell usage of it through their limited APIs.

It's not quite like that. The three main commercial providers which have their own data are Google, TomTom and HERE(which Microsoft uses). TomTom and HERE would be more than happy to sell it to you for a fee - it's their main source of revenue.

The data TomTom and HERE have is focused on navigation use cases - it doesn't have so much 'visual' stuff as OSM but has has large quantities of information that is only relevant for car navigation, e.g. historical traffic patterns and lane assignments.

Huh, I thought I remembered something about Microsoft buying HERE. Glad to hear that's not the case. I did also completely forget about TomTom - last time I used their stuff was on a Palm Tungsten T3... Amazing that they are still around!

Best is one of those loose terms that works well for marketing - it could be best because it's the largest set that's free-gratis & free-libre, for example.

Did you have a "best" in mind.

I'd definitely not consider plain satellite imagery to be map data.