The answer to your tittle question is because it hasnt been implemented. You can try to post it as a suggestion in Suggestions & Ideas forum section or there is a mod that allows you to move pieces: https://valheim.thunderstore.io/package/KG/Move_Piece/ or there is a free cam for building mod: https://valheim.thunderstore.io/package/gittywithexcitement/Build_Camera/

But I'd imagine creating a ladder and moving one would require the exact same amount of time since you'd have to highlight the ladder and place it again, so this would actually be MORE tedious if you moved the ladder but then realized you had to go back.

Just build the ladders you need and don't destroy them till the roof is actually done, there's a reason why real life builders and minecraft use scaffolding, which honestly would be better implemented rather than the ability to move and reuse a ladder. The only reason I could think of for this feature would be the resource cost to build multiple ladders, but they only cost 2 wood each so if you struggle with that chances are you probably aren't making a huge build anyway since a single thatch roof piece alone costs that much.