Why isn't the ovum reabsorbed into the body (like sperms) if it is not fertilised?
Apparently they are
Technically speaking, the ovum is not shed because it does not exist unless and until fertilization occurs.  Development leading up to the ovum otherwise stops before the final division of meiosis.
Now, you can ask why the body doesn't reabsorb oocytes, but then the answer is that it does, in a process called follicular atresia, with one or a few exceptions per  month.
Now what about that secondary oocyte that isn't fertilized?  Well, it's time to pull out a lovely article from 1917 (Harry Carleton).  Why are we so unable to match such prose today?  The paper describes a mouse study, giving quite recognizable descriptions of membrane blebbing and nuclear fragmentation characteristic of apoptosis in a decade during which it is usually said to have been forgotten.  (A 2005 work reports this for unfertilized human oocytes) Carleton likens the apoptotic changes he observed to atresia in the ovary, and says that the oocytes are reabsorbed by phagocytic cells.