Rusting of shower drains

After years of my earning, I bought a pretty good house from a real estate broker 2 months back. It is a double-storeyed house with bath attached bedrooms. Although it is not that old house, there are certain issues with the bathrooms. A few of the issues I repaired.

There was a leak in the toilet tank because of the refill tube going down into the overflow tube. So I held it above the top of the overflow tube by a clip which solved the issue.

Another problem was the rust in the shower drain. I wonder how easily the shower drains rust. Because of the rust, there was a foul smell coming out of it caused my child to vomit. Anyways I have to replace the shower drains. Being a plumber I had been to many plumbing drain services but till date, I haven't seen any shower drains that rusted for any house less than 2 years. What do you think can be the reason for the early rusting of shower drains?
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