It's a metaphor. People are trying to explain spacetime to you
as though it was a thing sitting in space (which obviously it isn't) because that's all the analogies there are. What else can we compare it to except some
thing? But a
thing (anything, any
thing) sits
in spacetime, and there the analogy breaks down.
What we
can compare it to, which is
not misleading, is the predictions made by mathematical models, which introduce no ontological claims about the things they're modelling: that is, they tell you how spacetime
works without making any claims about how it's like a rubber sheet, etc.