It's because raiding can be considered as serious business by most players and always brings endless drama to the players inside and outside the static group themself..

Here's a list of the dramas created by raiding:
  1. Someone is being singled out because of their performance.
  2. The treachery of whiteknighting those who are parts of the static despite they are core of the problems.
  3. Promoting "everyone can do what they want" is a blatant lie, because most will frutrate over losing in raiding, being casual raider is never an option.
  4. Being hardcore means being serious in everything, not my cup of tea.
  5. That someone in the static who's regurally absent/faulty/problematic but always have the first position in everything because that someone is a friend/lover with someone in the static
  6. Loot drops may cause jealousy and hate.
  7. A player having activity outside of the static group may be branded as a heretic (joining other group for raids or nin raid contents
  8. And many many more

And without static, raiding environment with strangers almost feels like meeting with a stranger who always claim "This works with my group, so if this doesn't work it's your fault" kind of thing.