There is an add on for that. They tell you when CDs are used. Let's be honest here--pvp is essentially a flow chart consisting of two decisions. If opponent is vulnerable, "go ham". If you are vulnerable, use a defensive. I find it funny that people try to pretend that PvP is that so much more difficult than pve. Only real difference is RNG. Sometimes, RNG is pretty much absent in pve on the defensive end. RNG doesn't require skill, either... Its all based on some guy's roll code.
Because that add-on is going to tell me the exact moment somebody is going to use any CD, right? Or, for that matter, any ability in their arsenal. PvP IS more difficult because it isn't JUST about "going ham". Every fight is different because every opponent is different. Not everyone thinks in the same fashion as everyone else. You can't make an accurate prediction regarding anything in PvP. However, you CAN make an accurate prediction in PvE. It will ALWAYS happen in the same way. Every ability will ALWAYS be used at the same time. Once you down a boss a couple times, that boss is on farm. Once you down a player a couple times, that player may go back and rethink of ways to change their rotation or how they react to certain situations.
Here's a sport analogy for you. PvE is hitting against the pitching machine. PvP is hitting against a live pitcher.
Also time isn't everything. By the time 2.4 (TBC) came out most guilds still hadn't cleared 2.3, which they needed to clear and farm before they could start with 2.4. And by the time the pre-expansion patch came out almost no guilds had cleared 2.4.
5.4 was cleared in like a week or something? So almost the entire patch is spent killing bosses you've already killed in different difficulties.