L21-30 Raikou is obtainable, but only from an event 8 years ago that was always guaranteed to be shiny, and all distributions of that event come with a ribbon that specifically blocks the Pokemon from going over GTS or Wonder Trade (so if, for some reason, you still have one and migrated it to a modern game without ever having given it a single level-up in all the years since, it still won't let you redeem the trade with that.
People who request such things may be aware of the history of previous events but unaware of the tradition of GTS-proof ribbons. Their reasoning may go something like "The GTS doesn't let you specify whether the Pokemon that people trade you is shiny or not. But by choosing 21-30, instead of any other level range, the only copies of those Pokemon that were ever released within that range are the shiny ones, so it's like I do get to specify that after all!" Never mind that it will never get cashed at all, which is up to them to learn firsthand.
As reasonable trades (such as one version-exclusive for another) tend to get snapped up quickly on GTS, the ones that linger around are unreasonable trades asking for something blatantly overvalued or even outright impossible to satisfy.
Note that some people have been able to offer level 30 Raikou, Entei, or Suicune on GTS. As it has to lack the ribbon in order for the server to accept it anyway, the fact that it's on GTS with a level under 40 is proof that it must have been hacked in some fashion. Sometimes you can even spot more inconsistencies, such as not being shiny or not being in a Cherish Ball.