Rat-free toilets

Once a week. That's how often the institution for pest control go out to deal with rats coming up through the toilet of someones house or flat. (anticimex)

It would be so easy to create a toilet that is completely free from that horror. It should be the norm to create toilets which has got a "flap" that goes up when you flush, for perhaps ten-fifteen seconds, and then stays shut the rest of the time. Why doesn't these toilets get made? It would be such a small thing, but very valuable for most people who doesn't like the idea of having a rat biting your ass off while on the toilet.

Why oh why? This is driving me crazy. This is ALL I think of, night and day. It's so exhausting, no one can understand just how exhausting it is.