While it is never expressly stated in any of the books, I would hazard two guesses. The first is hinted at in the series, mostly in The Long Cosmos. The second is more inference based on the viewed "rules" of stepping, rather than tied to specific statements by any in-universe experts:
1:In The Long Cosmos they discuss that the act of stepping is, fundamentally, an act of Imagination. A human/sentient needs to be able to visualize another place/the possibility of another world, and therefor is able to step. Though it's not as clear-cut as all that, because there's a lady who lives on one of the low earths, but can only see/hear the neighboring low earth. (I don't have the book handy so may be mis-remembering the earth number) Essentially she's blind and deaf on Earth West 22, but can hear and see her grandkids playing on Earth West 21. 73
So it could be that if a human could IMAGINE themselves stepping into a solid, they would. But nobody can bring themselves to believe in that hard enough to actually DO it, because they'd appear completely buried, without air, and with no way to get out. Likewise when they're stepping into another long earth and blocked by, say, a tree, they don't step because they aren't believing they'll step into a tree, they're believing they're stepping into an open place and thus the step doesn't work.
2: This one doesn't have any book dialog supporting the notion like the other, but it's another solution that makes sense to me. The steppers atoms need to displace the atoms previously occupying the space. Since solids have atoms that are tightly bonded together, they cannot be "pushed aside" as easily as liquid or gas. Though the "push" still happens, so for instance things that are solid but flexible, like grass, might be bent out of Stepper's way. This has the advantage of being a workable solution to "Why can people step at all when grass is everywhere and never going to be the same height." There obviously is some upper limit to the "force" exerted by a stepper though, as you get "stepped into grassland/snow" all the time, but "filling your living room with furniture" is stated to be an adequate defense against Steppers appearing in your house.
There's also no real reason why these two things can't be taken together, lord knows that's how I've viewed stepping ever since I finished the series! 209