Easy:
1) because your local store doesn't have it (stores run out of things like the specific shampoo or deoderant or detergent all the time, e.g. probably 2/3 of the time they have regular Tide but not Tide Unscented)
2) because depending on the item, it can be 30-50% cheaper on Amazon (drugstore prices in Manhattan are highway robbery), or
3) because you need another $5 or $10 item to meet the free shipping threshold, so might as well stock up on Q-tips because you'll need them eventually.
Also, expired/fake items are extremely rare, despite the media attention. In about a decade of ordering many, many hundreds of items on Amazon, I once received a counterfeit camera battery (which was quickly refunded) and once received a jar of nuts that was close to, but not at, its expiration. (I also try to order items sold by Amazon directly, and I've never personally been hit by a comingling issue.)
So... that's why.