Why so long?  I work with Oil on Canvas so I have time to get it right before it dries.  First, before I lift a brush, I usually research, design and draw what I intend to paint.  Then I paint the first layers on the figures in the work.  This is underpainting which takes some time and has to dry thoroughly before I add detail or glazing.  And many times it has to dry again so I can get the details right.  Then, after more drying, I paint the background around the figures (slow, detailed work).  Occasionally, I paint the background first – faster painting, but needs more drying time and often requires more layers of paint on the figures.
    When I add gold leaf (gilding) to a painting, the paint has to be totally dry so the gold doesn't stick where it shouldn't be.  That can take weeks.
    Note, I live right on a main street in Los Angeles, so I have to take special measures to protect my paintings.  Traffic particles, dust and hairs collect on the work and have made repainting necessary.  So I hang my work with a clear cellophane cover to control that, and on larger paintings (Girl Reading Pirates), I use food wrap to protect the parts I'm not working on when I draw, paint or guild and still use the cellophane, also, while it is drying.