A biopsy is when they find a lump that isn't supposed to be there, so they take a tiny bit of the lump out to see what it's made of (especially to see if it's cancer). And the if it's cancer, they go in again and take the whole thing out and maybe blast you with radiation too.
So why not take the whole lump out the first time they go in to do a biopsy? It's not supposed to be there even if it isn't cancer. If it is cancer that just needs a lumpectomy, hey, guess what, you've cured it! All done, off you go, have a nice day! And if it's cancer that requires further treatment, you're going to need to go in for further treatment anyway.