It is often necessary to bring the rectus muscles together if they are splayed apart following pregnancy or weight loss, however it is not always needed. If you have a bulge between your six pack muscles when you do a straight leg raise while lying on your back, then your rectus muscles are diverged and need repair. Not everyone has this divergence of the muscle and so not every needs them to be stitched back. Stitching the muscles is painful and can bring with it potential complications and so should not be done unless it is required. Are you happy with your result? If so - don't worry about whether the muscles were sutured or not.
Separation of the muscle (diastasis) is treated by suturing the muscles during abdominoplasty. However not all patients have this problem. If you are dissatisfied with your results you might consider rediscussing this with your treating surgeon or seeking the opinion of another a board certified plastic surgeon.