The trampoline concept is a 2 dimensional representation of a 3 dimensional phenomenon. The way to look at that image is that it says gravity gets stronger the deeper you go in the well,.and it's harder to escape. It's actually acting like that all around the massive object. Its called an "analogy", which is a way of describing things that cannot be directly visualised, like spacetime warping, so, scientists try to use familiar things that behave in approximately the same way. You shouldn't take them literally as being an exact description or representation.