One issue that may reduce the effectiveness of an improvised tool is that the outside water pressure on the window stresses the glass in a way that makes it harder to break from the inside. You have to overcome that water pressure to some degree just to bring the glass to a point of neutral stress, then go beyond that to a breaking stress in the outward direction. That effect would work against a broad force like kicking the window. The focused force of a window-breaker tool can impart a breaking stress over a small-enough area that water pressure is negligible, yet that small break will propagate until the window shatters.