Do you mean they have no collisions? Some older meshes lack collisions, while most of our other meshes just do.
Grass patches and other foliage sounds like a character creation glitch, where you randomly collide with invisble "walls". Easily fixed by resizing your character and joining back (afterwards you can size back up/down if you want, but you need to at least resize, get into the game once, and then it should be fine from there)
But as well, some foliage meshes in general just have collisions, period. I know most, if not all, of the meshes in Default have collisions
Hope this helps!