Too Many Decks
The Casino's were forced to start using multiple decks because of the successful onslaught of counters back in the 60's...I don't think they realized at the time that it would also give them a greater edge...If they had, I think they would have done it before.
In a one deck game, and assuming that no face cards are showing, if you have an Ace in front of you and, for purposes of easy fractions, there were 50 cards left in the deck, your chances of getting a blackjack would be 16/50 which is 32%.
With a six deck shoe if there are 310 cards left in the deck, your chances are 96/310 which is 30.96%...This type of favorable condition for the Casino also applies to double down situations...These are part of the reason that a 6 or 8 deck game is more difficult to beat.
For a non counter, and if the tables are slow enough to allow you to do so, you should only play for the last three or four hands before the cut card...That might reduce the Casino's edge down to 0.20%, but that reduction is really just an educated [hopefully] guess on my part.