Because its a pirate site. Buyer beware.
Generally speaking, any third party retailer can offer discounts and turbotax is prohibited by antitrust law from setting a minimum or fixed price. Retailers can even sell below wholesale cost if they want to attract customers or grab marketshare from a competitor. But this particular site is not authorized. They are selling pirated software at best, or malware at worst.
There is no public official list of authorized resellers and I don't know why. But I have reported this seller before. And any reseller that sells for less than Costco should be immediately suspect because Costco sells below wholesale cost (because they make their operating profit on membership fees.)