Why aren't open wounds re-stitched 3 weeks post op? (Breast Reduction/lift) (photos)

Thank you for your questions and photos. 
Delayed wound healing is a potential completion of breast reduction surgery. Open wounds are often allowed to heal by secondary intention with local wound care, and the process to complete healing is slow and can take weeks to months. Once healing is complete, the scars may be revised. 
Your photos show open wounds, with mostly clean bases. Signs of infection would include chills/fevers, surrounding redness, increased warmth, and/or foul-smelling discharge. 
Antibiotic therapy may or may not be necessary and depends on whether or not active infection is proven or suspected. 
In these instances, the best course of action is often to follow up closely with your surgeon so that he/she can monitor your progress and make changes in treatment as needed. Best wishes!

Suturing these wounds would likely lead to re-dehiscence.  Also it would change the breast shape for the worse.  It is best to allow these to reepithelialize.  Keeping them moist with an antibiotic ointment is the best wound care.  

The plastic surgical standard is to allow dehiscence of surgical wounds to close/heal by secondary intention. Because the edges of the immature skin growing are not strong enough (having the collagen, elastic tissue strength to hold the pressure forces to keep the edges together. Thus the wound would reopen even larger). Sometimes a wound vac apparatus aides in speeding the closure time. Best to discuss with you chosen operate surgeon. 

Your wound does not appear acutely infected, as not all open wounds are infected. Infection would be indicated by drainage that is smelly or pus-like, surrounding redness/swelling and/or fevers. Having an open wound and absence of infection markers in your labs definitely does not require antibiotics. Unless large, gaping or infected, wounds are usually kept open so they may heal naturally from the inside out. If your surgeon were to re-suture these areas closed, it could potentially close in bacteria and you can easily develop an infection. It is almost always better to allow the tissue to fill in (the red areas in your photos above), the skin to grow over that and to do more superficial scar revision surgery later if needed. The process to close can take a pretty long time sometimes, so you should put yourself in the best position possible to heal quickly - make sure you're not using any nicotine products, that you have a healthy diet with enough proteins, vitamins and minerals, and that you're not overly stressed out.