"If we did this... we could instantly kill Thanos with no trouble whatsoever. But, it would be such an anticlimax, what with all the buildup Thanos has had, that the fans and movie goers will be incredibly disappointed and irritated. That will result in a drop in sales and brand loyalty, which might very well result in the end of the Marvel Cinematic Universe itself. What future is there in which we defeat Thanos, but with pizazz?"
...
Honestly, I just figured Thanos had some sort of protection against instant kills like portal slicing, either due to weird titan toughness or just using his infinity gauntlet.