Planetary take off and landing
I checked, and the book specifically states that even a planet's atmosphere extends outward the same distance as the planet diameter.
That would be 8000 miles for Earth, or about 41 days to get out of it in the fastest Spelljamming ship. Someone at WoTC didn't do the math.
Even using real world distances (60 miles) that's 20 hours for the slowest ships and almost 8 for the fastest. A lot for one helmsman, and changing helmsman would be....risky, what with the period you'd be falling and all.
I've used the size categories from the previous edition, and Earth's actual atmosphere size as a guideline to come up with atmosphere envelopes for planets:
Size A: not planets. less than 10 miles across envelope is quite large, gravity plane is flat
Size B: 10-100 miles across, 15 miles
Size C: 100-1k miles across, 30 miles
Size D: 1k-4k miles across, 45 miles
Size E: 4k-10k miles across, 60 miles (Earth)
Size F: 10k-40k miles across, 75 miles
Size G: 100k-1000k miles across, 90 miles (Jupiter, all atmosphere in reality)
There may well be exceptions, of course: a planet that is a ball of water may have very little air around it. And a planet that is all air is going to be thousands of miles of nowhere to land.