Your statement is wrong, so does most of the replies..
As far as dealer upcard, 6 is definitely better for player than 5..
Dealer will bust more with 6 instead of 5 in any game except s17 single deck game-which are virtually zero. Bust probability is same in s17 dd game. Rest all games,6 bust more often than 5..
I think you are confused with EoR numbers::
5 has higher EoR because Dealer can never bust with '5' while '6' is good enough to bust dealer hard 16.. Same goes with double on 11
'5' will mostly make dealer weak up card into a strong hand enough to beat most of your stiff splits and double downs..
Yes, there are various other reasons too..