Did you ever play at a high level in those "so called MMO"? Maintaining a rotation != spamming skills. But if you only know XI and XIV, this concept may be a bit harder to understand.
On top of having an order to execute spells/abilities, like A > B > C > A > D... You also have a priority order list to follow. A/B/C/D may be the four "damage" abilities you will use the most, on top of that you will have other things to manage. Like refreshing DoTs, refreshing Buffs, etc.
This can become extremely complex because of procs that doesn't occur in a steady manner (i.e. : X% chance of your next cast to be instant). And also because of battle mechanics that will force you to skip certain spells and will disrupt you in your rotation.
Here is an example of a 32 seconds rotation in Rift, for a Mage DPS. You don't know the spell names/abbreviations, but it doesn't mater.
(IC) (/IC) is an ability that consume charge over time (see it like if you were depleting your TP in XIV, gradually over time). When it's on, you do more damage. Since the supply of charge is limited, you have to keep activating/deactivating it only at specific part of your rotation to maximize DPS.[FB, (IC), CD, ^SV, Def, ^DT] (/IC) <2s> VB
[FB, (IC), CD, ^SV] (/IC) <4s> VB
[FB, (SL:D), CD, SV, Def,^ DT] <3s>
[FB, CD, ^ SV] VB <4s>
The rotation is so complex that DPS between two mages, having the exact same gear can vary by up to 30%, and that's on a battle dummy (so with no external factor disturbing them). This is a typical example where skill > gear. Someone with gear that's 1 tier lower could out-parse someone with better gear.
Maintaining a perfect rotation in a "real" situation (in raid) will be even more trickier. Only experienced players who know an encounter perfectly well can maintain a high DPS.
Last time I checked, current XIV "rotations" (hard to even use that word for XIV) are extremely easy to follow and the gap between two players, with the same gear, if we had battle dummy, would be non significant. We do rotate spells in XIV not because we want to, because we need to due to high CD.