DDO has real-time combat. So it is possible in an MMO. Why can the devs implement it in ESO?
The developers could use a real-time combat system in ESO but real time combat can be a huge issue. As a true real-time combat system in an MMO could be a huge exploit. For instance if your say a rogue or a warrior. You get up on a caster, who suffers a timer reset of a couple of fractions of a second when hit. That same character often has to stop and be facing their target while attacking. Then wait a couple of seconds to get their spell or attack off. Basically you could put that character in a constant loop which allows them to never get an attack off. By just running around in a circle and spamming 1 button. Which has nothing to do with skill. It has to do with the classes mechanics.
So say you fix it so the caster doesn't have to stand still and wait 3 seconds. So now what happens to your warrior or rogue if that caster can constantly run and stay out of your melee range. Yet be able to hit you with powerful spells that kill your melee guy in 2 or 3 hits? And those hits are in real time. This is probably why most MMO's don't use real time combat. They use a universal timer. Which limits players to so many attacks in a set amount of time.
In most console multi player pvp situations almost everything across the board is even. Same console, same controller, same voice communication, same character , even access to the same gear etc. Maybe your connections speed is only the real difference. So when it comes to pvp, skill can be the deciding factor.
But a computer MMO has way to many variables. Different computer set ups. Different control systems. Different classes. Different builds. Different equipment. Different levels. Different connection speeds. Different player types. Most players never notice the differences unless they are in pvp. Then the differences can really matter. In fact sometimes these differences can be so huge, that skill can be the least important aspect of pvp. Many will laugh at this. Though here is my real life example...
I'm a carebear player but my friends are pvp guys. They always drag me on pvp servers. I personally don't see it as hardcoe. As I can pretty much pick when I want to pvp most of the time even on such a server. Just by picking areas I know to be safe to solo. When grouped though my friends and I usually roll enemy groups in open areas. But the point is this. I usually have a low end computer, with a basic set up and I refuse to pay real money for in game advantages. My friend on the other hand is a hard core pvp guy. Has been highly ranked on a WOW server and has a top of the end rig, with gamer mouse keyboards, and buys in game advantages with real money. For instance he paid a farmer in WOW to pvp for him all day. While my friend was at work. This kept up his points and got him top of the line gear. While I only got pvp gear I earned from my actual pvp. So my friend loved to duel me all the time. We were the same class. And we had different favorite builds. Though through end game raiding, I had some decent gear. While he had top of the line pvp gear and end game raid gear. He would beat me in a duel every time. And we would all laugh about it. But then one day, another friend wanted to try an experiment. So my friend and I traded computers. I was able to win about 40% of the duels with my character on his top of the line computer. But when I played his toon with his top of the line pvp gear all of a sudden I won all the time. Then when I learned his macro setup on his gamer mouse and keyboard, it was mind numbingly easy for me. My friend is a much better player and pvper than I am. But in the end his superior set up and gear made even me able to win all the time.
So if you added a real time combat to your average MMO pvp system. The problem could get even worse. Both in boredom for those with real skill and real good computer rigs. And make those with no skill or weak computers even easier to roll.