Theoretical vs. Actual Pot Equity

Here's my I-hate-myself hand of the month:
(In the same breath I ask myself how people can play so much "better" than me?, and also how can people spew so much????)

hand #1
9 handed $20/$40 LHE
Limper1 is unknown
Limper2 is a loose tricky player who slow plays big hands, but is also capable of making moves
Limper3 is unknown
Limper4 is a LAG
Limper5 is a semi-LAG
Hero is in SB
BB is a TAG
preflop: 5 limpers and a couple of muckers, Hero completes with Ah9d in SB, BB checks (7 players, pot size 7 small bets)
flop: 9c5h3d, checked to L4, L4 bets, L5 raises, Hero 3 bets, BB mucks, L1 cold calls, L2 cold calls, L3 mucks, L4 and L5 call (5 players, pot size 22 small bets)
turn: 9c5h3d2d, Hero bets, L1 calls, L2 raises, L4 and L5 muck, Hero tanks and folds, L1 calls (2 players, pot size 16 big bets)
river: 9c5h3d2d8h, check, check, L1 tables 6h3h, L2 tables 5d4d and scoops.

Hero is getting 15:1 immediate pot odds, not closing the turn action, and is out of position with little chance to measurably improve.

I had no history with L1, but at this point in the hand I can put L2's made hand range as: 33, 55, A4s, 46s, 53s, 95s. His semibluff hands are going to be straight/flush/pair combinations.

Assuming a range for L1 of A5,A3,87s,76s, 64s; Hero's blended pot equity on the turn is less than 5% or less than 0.75 big bets. How can I call in this spot? (Hero's actual equity on the turn is 54.75% or 8+ big bets; GEEZ! What a huge divergence between "theoretical" equity and actual equity!!!)