The Illustrious Class aircraft carriers from the Royal Navy had armored flight decks, and I've read accounts of bombs barely scratching the paint on the deck when it is detonated and kamikazes bouncing off the flight deck without doing significant damage.Meanwhile, USN carriers didn't have any sort of armor on the deck, which was completely vulnerable to bombs and Kamikazes. The USS Franklin was almost destroyed because a bomb had penetrated the flight deck and detonated in the hanger. It wasn't until the Midway Class of CVs did armored decks were introduced. Why did it take so long for the USN to introduce armored carriers when they proved their value in the Royal Navy?
The Yorktown and Essex classes were too early to take advantage of the WW2 lessons of the RN. The Yorktown was launched about a year before Illustrious was even ordered, and was commissioned about 2 1/2 years before Illustrious. Illustrious was commissioned in May 1940, and it was almost a year before her armour was proved in action. On the 7th of January, 1941, Illustrious was heavily damaged by multiple bombs, including a hit on her unarmoured aft elevator, and a hit that went through her armour and then exploded. Formidable's severe test came on 26th May, when she was hit by two bombs.
Thus, it was in early 1941 that the value of armoured flight decks was demonstrated. This was long after the Essex class had been designed, and about a year after Essex had been ordered. Essex was laid down between the above-mentioned two bombings. Simply, the Essex class was too early to benefit from the RN's WW2 experience with armoured decks.
The USN was already exploring designs with armoured flight decks in 1940. Lessons learned from the bombing of the Illustrious were incorporated into the design work - not the concept of an armoured flight deck, since that was already in the design, but compartmentalisation of the hanger deck, and better damage control, in light of the severe damage to Illustrious's hanger. This was the design work that would lead to the Midway class.
Of course, armoured flight decks don't come without costs - the USN estimated that the aircraft carried would be reduced by about 10% in number by the adoption of an armoured flight deck. A simple brute-force solution to overcome this was simply to build larger ships, and the Midways ended up very large ships. However, this meant that more of the final Midway design was new, and the design process slower.
Midway was ordered in August 1942, and laid down about a year later, and commissioned about two more years later, in 1945, just after the end of the war. Thus, we can see why the USN didn't introduce carriers with armoured flight decks in time for WW2 service: there wasn't enough time. It takes time to design novel ships, and time to build and commission them. The design work that led to Midway was already underway before Illustrious was bombed, and the lessons from Illustrious were heeded. But this was already in 1941, and Midway wasn't ready for service before the end of the war.