Cause that's what the places that sell them at saying
Like many larval insects BSFL put on fat as they age, this is their energy for pupation and the non feeding adult stage. Diet also plays a role. It not inaccurate to say that a small or medium BSFL has a low fat content, though by the time it's ready to pupate it will have gone way up.
Crickets are pretty low fat as far as common feeder insects go. Even the beloved dubia roach is fattier than adverts would suggest.
Personally I don't suggest BSFL as a replacement for crickets or roaches, but as a part of a more varied diet. I feed roaches, BSFL and super worms regularly; silkworms and horn worms when available. BSFL have the benifit of a different nutrient profile from other bugs, including good innate calcium
hosphorus ratio (it's not well understood how well this is metabolized, but it's also not well understood how well dusted supplements are metabolized). They have the draw back if not having a hard shell (soft food diet is implicated in poor dental health).
