The substrates aren't just floating free, in many cases they're collected by binding proteins, which form parts of protein complexes, and these protein complexes are guided around the cell by it's cytoskeleton. In most cases when substrates are unavailable the protein complexes maintain the enzymes in an inactive state.
Don't think of cells as disordered bags of water. They're more like highly ordered liquid crystals.
Don't think of cells as disordered bags of water. They're more like highly ordered liquid crystals.