how do people get away with this

50 left in 18k superstack early antes.  I have 88k at 1200/2400.  Villian (60k) opens for 7k in the HJ and I 3bet AQo from the cutoff to 20k planning on calling his shove(he's bad).  Obviously he flats 1/3 of his stack.  Flop is 865.  He checks. I put him on a hand like KJ and with 45k in the pot I pushed him in for his last 40k.  He calls with 78.  I turned an A giving me the lead but the river 9 gave him a straight and crippling me.  K8<AQ a few hands later.  I hate poker.