Snapdragon, I guess I have always thought of Dream Valley being in a magical land of its own and not of Earth. I don't know if that means magical like Narnia through the wardrobe, an alternate dimension/reality, or another planet. In my fanfiction that I've written, there is a magical portal in the mountains surrounding Dream Valley that connects to another portal in the mountains near Megan's home. So like when Firefly and Megan go up into the clouds in "Midnight Castle" and come down in Pony Land, in my imagination they seamlessly passed the border between two worlds while shrouded in the mist of cloud.
For my crossover, I even thought I could have the ponies tell Garnet, "Hey! We can get you to Earth quick! We got this portal thing and we go there all the time!" However, I decided not to bog down the story with that as I have to finish this soon or I'll lose interest and never finish. I decided the ponies will just have their own way unsaid for Megan visits, and just let them leave to join the Gems for this fight on the ship.
it makes sense to me, they've always had to go 'over the rainbow' to where Megan lived I kind of figured it was also like RainbowLand in Rainbow Brite, its on another plane of existence/another dimension and they had to travel across something to get to earth.