Muscle activation is highest in the first half of the lift when your muscles are at a mechanical disadvantage and have to produce a ton of force to overcome the weight. The mechanical advantage increases in the second half of the lift AND your muscles are aided by the momentum of the weight which ultimately lowers muscle activation in the second half of a lift. Using accommodating resistance methods can increase activation during the second half of a lift!
Reference: Elliott, B. C., Wilson, G. J., & Kerr, G. K. (1989). A biomechanical analysis of the sticking region in the bench press. Medicine and Science in Sports & Exercise, 21(4), 450-462.