They can! That's a different process called importing, and people do that a lot. For example, I've imported all of Walmarts in the US a few years ago by request. See OSM wiki on imports: https://wiki.openstreetmap.org/wiki/Import
I should add that when you add or edit a bench in Every Door, it shows the backrest and the material attributes at the top. So hundreds of benches in my area have these mapped :)
I'd say that is closer to what Maps.Me / Organic Maps do than to Every Door. The latter has a huge focus on updating amenity attributes, which involves doing away with the interactive map for the main UI element. In a shopping mall, for example, StreetComplete's view would be cluttered and mostly unusable, while Every Door is perfectly fine for adding or confirming every shop.
Absolutely! I agree with another user that you should switch to Organic Maps, but if you want to do more mapping, Every Door helps with that. It uses iD editor presets, so you can contribute even without knowing OSM tagging, although that would help.
The story of mapping is different for every corner of the world. E.g. in the blog post there's a pic where we mapped the city approximately due to absence of satellite imagery. In this post https://www.openstreetmap.org/user/Zverik/diary/399925 it shows that some regions like Africa were nearly empty ten years ago, before the Humanitarian OSM Team lauched their operations. And in 2012 the focus was mostly on basic map features like roads and rivers, while now we're adding final building outlines.