scribus always converts the text to shapes.
that's the only way to make sure that the text will look exactly the same as in the original pdf.
and since scribus is a layout program and not a pdf editor this is the right thing to do.
but, indeed, it would sometimes be nice if scribus could get the text from a pdf. you will probably lose most or all the formatting, but you will have your text in frames that are already at the right place.