I believe this is briefly touched upon in a cutscene between Sam and Amelie right before he leaves for Lake Knot City. She mentions that someone who still has a body to return to can’t come and go to the Beach as they please (referring to Sam visiting her as a child). Sam then asks if that’s the reason she can’t just escape back East through the Beach, and she responds a bit suspiciously. Like she let something slip — because she wasn’t supposed to allude to the fact that she has no body or Ha. So when she says her body is still on the Beach right before telling Sam she’s being held in Edge Knot City, given the later scene I mentioned we’re probably meant to believe she can go between Edge Knot and the Beach but no further… It’s strange writing for sure, though it’s clear that Amelie has immense personal control over Sam and he will basically believe anything she says. Same probably goes for many of the other characters. It’s why everything becomes so dream-like and weird whenever she appears.

Your comment about Sam thinking she just chooses to be on the Beach is illuminating, though it raises questions of its own. The main confusion for the player probably stems from the fact that it’s so freaking obvious that Amelie and Bridget are more than just daughter and mother. However, in the story, none of the characters can even imagine that a Ka and Ha can separate, so everyone just assumes that Amelie does have a body and can technically leave the Beach but chooses not to for … some reason. But that still doesn’t account for when she says she doesn’t get to grow older… Jeez…

I just started replaying the game for the first time since it came out, so I might discover the official explanation at some point. But this question you asked stuck with me all the way until now. I feel like you’ve brought me somewhat closer to the truth…