PICT files are object-based files. That is, they can contain distinct geometric objects, such as lines, rectangles, circles and polygons. Another type of object that a PICT file can contain is a bitmap object.
When you save as a PICT file from a bitmap / paint program, such as Adobe Photoshop or GraphicConverter, you're saving out a PICT file with a single bitmap object. You're not saving out a PICT file that contains distinct objects. Since the entire drawing is a bitmap object, it will only be editable with bitmap editing tools.