Because the BLM burn strategy is a really poor method of killing Garuda and is no where near as 100% as killing a set of sisters and moving on. We can get Garuda to the same HP level as 4xBLM with only 3xBLM and we still kill sisters because it's more consistent and has less of Murphy's Law.

And, you don't do 20-25k on Garuda. Archer doesn't have the combo's available to push damage like a real DD would - even a DRG who does half plumes/half Garuda would have a hard time breaking 25k without a Gae Bolg. (22k Garuda, 4.5k Superna, 9-15k Plumes is pretty normal)

Crab Bow and AF doesn't one shot plumes, so you're just throwing what you feel out there and not basing it on reality.