Horizontal pressing exercises like bench presses and push-ups effectively strengthen the pecs when they're in their mid to lengthened range. However, they don't strengthen the muscles when the pecs are in their shortened range, like when the arms are straight out from the torso. In that position, you're no longer working against a force that's pulling your arms apart.
This is why you need to also perform cable pec flyes (one-arm or two-arm version) or machine pec flyes.
Cable Pec Flye, Unilateral Variation
Cable and machine flyes make you work against a resistance that's pulling your arms apart when they're extended away from the body. This is a range that's often neglected by traditional multi-joint pressing exercises.
Double-arm cable flyes do an even better job than the machine flyes because the cable allow you to cross-over your arms at the finish and get an even greater range of motion.