Probably they think its tubes are filled with contraband.

My view from experience now is this: ship it in a crate, the volumetric pricing of boxes that can fit a bike frame raise the "weight" to the point where a wood box with internal reinforcement provides the insurance at no additional cost. the common carrier insurance offered is just not designed to handle out of production, vintage, verging on antique items.

Nothing speaks "don't mess with this" more than a crate, not perfect, planes do crash...

Otherwise, ....are you feeling lucky today?