In reality this is not true, if a raid is fun it continues to be played even if the loot is no longer new or top. Raid trains are a reality. People play them for fun and for threads, not just for the items themselves. And in general there is always some useful item left in a raid that is no longer so new.
Not quite true. They run the "endgame" version of it if there is one, maybe. But by the time I was ever able to flag a single character for Vault of Night, for example, nobody was running it as a Heroic raid since it's a level 10 raid (what originally was the endgame), only as an Epic raid. And the characters who'd flagged were not at the level 20 yet be due to DDO NOT being my main MMO, and my having so many alts who all get played. I
could have flagged VoN when it was still relevant, true. Except I was incapable of soloing all the prereq quests needed to flag for the raid at the time, and nobody was running some of them. And by the time I
could solo them people were instead running Black Abbot instead.
By the time I (in theory) had the ability to flag for Black Abbot, nobody was running it because the Lolth raid from Menace of the Underdark was the new hotness. Could I have flagged the raid when it was still relevant? No, I couldn't. And that was because there were quests that REQUIRE multiple players trading a key item back and forth to complete the dungeon, and nobody was willing to run those quests. Which meant I had to start grinding favor on a second F2P account to unlock the various Necropolis packs on that account, just so I could try dual boxing those quests.
By the time I had a character approaching the ability to run the Lolth raid, nobody was running it anymore, because they were too busy farming the new Mists of Ravenloft raid.
Do you see the problems with a more casual player trying to get into raiding yet? Every time I've gotten close to the point where I could flag for a given raid, the player base in general had moved past it due to a new raid having been released. Or they now only ran it at the new level cap, which none of my characters were anywhere close to yet. Or now it's "at the level cap, and on high Reaper difficulty"
All of this is above and beyond the fact that grouping kind of died years ago in DDO. Oh, sure, people ARE running groups. But they're running static groups with the same sets of people from their guild or friends list, not forming PUGs. Pick up groups kind of died out around the same time the game went Free to Play, I think as a protest to all the new players flooding the servers and being obnoxious at the time.
As I recall, I'd ran into another "need a group with actual people in it" quest while plowing through the quests to flag for Black Abbot with Venoma once she was level 20, only to find it impossible to get a group for that quest. So instead I ended up farming Token of the Twelve fragments in the Sentinals of Stormreach chain and the prelude chain for Menace of the Underdark until Venoma could reincarnate the first time. Maybe once the devs FINALLY decide what the true level cap will be, I'll eventually get to a point where I can flag for, and run some raids. Not holding my breath though, since level 20 was originally the goal. Then they raised that to level 30. And now they've raised the end goal to 40, apparently? I expect a month or two after the cap gets raised to 40 another expansion and level cap increase will be announced.