I believe that some areas are blacklisted from spawning tappables due to safety reasons (and possibly other reasons). I've flown on several occasions and the only areas tappables will spawn at an airport seem to be within the confines of the terminal. I've also never seen tappables close to interstate highways.
EDIT:
From the MC Earth FAQ page (https://help.minecraft.net/hc/en-us/articles/360033744412-Minecraft-Earth-FAQs):
CAN GAME PLAY LOCATIONS BE IN PLACES THAT MAY NOT BE SUITABLE?
Gameplay in Minecraft Earth is generated in locations that exist in
public places such as parks and open urban areas. Gameplay conditions
in any area can depend on many factors, time of day and surroundings,
amongst other things. We encourage our players to exercise caution and
to not play in potentially hazardous locations.
If you find a game play location in a place that is not ideal for game
play, please go to http://aka.ms/mcearthsupport
It appears that at least one of the sources they use to determine what areas can generate tappables is Open Street Map:
HOW DO I GET MORE MINECRAFT EARTH GAMEPLAY LOCATIONS AROUND ME?
We use a variety of different inputs when we decide where to put
Minecraft Earth gameplay locations. One of the inputs we use is Open
Street Map data, which is a map built by the community of users. If
you’d like to see more gameplay locations around you, one of the best
things you can do is improve the accuracy of the Open Street Map data
in your area.
In the future, we are exploring additional ways to get your input on
where you can play Minecraft Earth – stay tuned!