McBeefsteak

Maybe in the same way that spren behave as expected the shard blades are simply meeting expectations. Since few records survived from before the recreance, maybe the only surviving records are ones referencing shards as swords. This would make the commonly held belief/expectation that they are all swords, and then they just would be. That or maybe it was a choice by the orders as a sword seems like a more "regal" weapon then say a spear or hammer.