“Why” is kind of a bad question The authors of Makie chose to have a Block that just draws a text and call it Label. Other Blocks are for example Axis or Button or Textbox. You see Makie is half of a UI-Toolkit with which you can build quite interactive applications. Here is a list of all Blocks.
So you see a Label is not really meant for usage on a graph. It is it’s own indepent thing. If you just want to annotate a graph, i.e. but some text somewhere on the plot, then Label is not what you want to do. For annotation you should use the function text!
Label is really just a wrapper around one text plot, with the wiring set up so that it’s dimensions can be communicated to the layout. This has overhead though, so you wouldn’t want every text in your plot to be a Label.