From the Cryptic Blog:
"Traditionally, there have been two dominant design motivations for instancing:
[...] Secondly, in the early days of MMORPGs one of the ongoing problems was
players camping valuable spawns or drops, effectively locking other
players out of content."
And the devs of the various MMOs weren't having players doing that!
Not when they could create raid instances and lock the majority of players out of the content themselves. Why should the gankers and campers have all the fun?