Why is there 500 mg vitamin C tablets or even 1000 mg, if the recommended dose is less than 90mg/day?

Because these tablets are aimed for people who are vitamin C deprived so they'd need more C. Plus you'd need a lot more then 200mg to O/D on vitamin C. Also just because the tablet has 200mg of C, your body loses some when metabolizing it and doesn't absorb it all.