On the planet I come from (autism world) a lack of independent play is treated as if it were a skill deficit. If the child won't play by himself (mine sure didn't) you teach him to, by rote, first a few minutes, then gradually adding time. Legos, K'nex, all that sort of thing was taught and his participation rewarded as if it were an unpleasant task to play. Which it kind of was, for him, until he got better at it.
At 8, he's still not fabulous at managing unstructured time, but he's better at it than he used to be.
I am fascinated that your non-autistic children do this-- I really thought it was just our neurological issue. I wonder how common this is among gifted kids.
DeeDee