you're going to open them

Not necessarily. They're rare enough that the average player who tries to go F2P for a bit or only invest a small amount in, or only plays for 3-6 months, could easily never see one.

I've put significantly more time and money into this game than the average HS player (though maybe not so much as the average /r/Hearthstone player) and I think I've seen 2 golden legendaries in 2 years. Jaraxxus and someone bad from TGT.

The fact is that unless people are opening an extremely large number of packs it doesn't make sense for them to assume that an extremely rare event will happen for them when trying to gauge how much return they should expect.

If the Powerball jackpot gets over $585,000,000 then the average payout for a ticket is more than the cost of the ticket. But that doesn't mean that a person should buy a ticket under the assumption that on average it'll be worth more than the $2 they paid, because for the vast majority of players it won't be.