Why is simulation effective bandwidth of LM741CN op-amp wider than calculations says it should be?
My question is why is this happening?

You are probably using an AC simulation to generate the Bode plot.
The AC simulation method assumes circuit linearity, therefore it neglects the slew rate effect.
As to why you don't get the full 67.8 kHz you calculated under linear assumptions, probably there is either some loading effect due to the relatively low (for a 741) 1 kohm load on the first stage, as well as some compounding effect where the net gain of the two stages drops by more than 3 dB before either of the individual stages drops by 3 dB. The LM741 model in Multisim might also have different values for \$f_t\$ and \$A_{CL-O}\$ than you used in your calculations.