I voted 'they just are', although I have seen some that are definitely very nice to look at and some that aren't. It varies.

And I pray that I fall into the 'nice to look at' category.

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Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these