Why do writers do this?

Bring back villains and heroes OTHER writers created and killed off in their own legendary runs?

Or villains that were made legendary by their deaths.

Examples:

Korvac Skurge Malekith The Green Goblin The Lord Papal The 12 gods of the Instrumentality

Thanos and Warlock are a little different as it was Starlin who killed them...but I still would have preferred they stay dead.

Now characters like Ultron, Magneto, or Doctor Doom you expect to come back......

Don't the writers who go 'Hey! That was a cool character! I'll bring em' back!' Realize they're cheapening the original run?

The writers aren't the ones to be blamed; I blame the editors. Editors are the ones who control continuity. Many writers may like certain characters, or find story arcs easier to fabricate with them. It's therefore up to the editors to authorise the changes. If you look at fans who clamor for changes, you realise many of use would do the same things the writers do: Undo mistakes, write stories to suit ourselves, and generally tick off those who disagree with us .

P.S. Usually the writer who kills a character didn't create him. So why should he have the last say?

Obviously the previous writer failed to tap the true potential of said villains, potential the current writer is uniquely talented to tap.

Yes, blame editors. Blame editors! (They're not a real country, anyway.)

Simple lack of imagination and talent.

Yes, blame the editors. The buck stops there. Can a writer resurrect a character without editorial approval? No. If you have a problem with retcons and resurrections, see the people who allow it.

Skurge came back?

-Ralf Haring 'The mind must be the harder, the heart the keener, the spirit the greater, as our strength grows less.' -Byrhtwold, The Battle of Maldon

Huh. Tell the current X-Men writers that.

Now you've got me curious. Who are Lord Papal and the 12 Gods of the Instrumentality?

One of the recent Thors

Wasn't it Claremont's idea to kill Pheonix?