because my time is very valuable. because no sewing machine can load a bobbin with as much as you'll get from a prewound. because winding bobbins is annoying. because i wanted to. because i could.
that's why.
i haven't needed to buy them in a while, but the prewounds i bought were only pennines more than the cost of an empty bobbin. it was good 100% cotton thread. overall, i think i came out waaaaaaaaaaaaaay ahead.