They're completely different niches. Blow molding helps you make a million of a thing, 3D printing helps you make a million different things. There's great for low volume items without significant strength requirements. That includes prototypes but also anything niche - like parts for blow molding machines.

But blow moulding machines big enough to make a drone shell are massive.

If you had suggested a more portable technology like vacuum forming maybe you would have a point, but blow moulding is definitely one of the more expensive and least portable ways of forming plastic.

Is it? With a 3d printer you can print any piece on demand, change design without changing tooling and make pieces that are semi-hollow inside for lightness.

Edit - should add, early in the war Ukraine was crowd-sourcing drone parts from citizens with 3D printers at home. This likely grew from that.