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.

Hello Opus 17. Are you a representative of Intuit?  Just curious how you know that GreenTaxExpert is not authorized and that the software is pirated at best or malware at worst.  I found the greentaxexpert site via a Google sponsored ad.  Would google allow an ad linking to a site selling pirated software or malware?  Thanks

Are you kidding?  Google will take money from anyone.  You think Google contacted Turbotax and asked their permission?

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.)

How kind of you to dodge the question, Opus. You could just be straightforward and honest and say that you have no idea if they're legitimate, you're making a guess. Costco makes money off their sales as well, they don't pass items on to you at cost. Please keep your comments to things that you know. I have no idea if this site is legitimate, your post simply betrays that you don't either.

I used it last year, no problems.