Why spermatogenesis is an independent process while formation of ovum is dependent on fertilization by sperm?
This can only be answered logically. Imagine bellow paragraph.
If secondary oocyte does not require fertilization for completing 2nd meiotic division then it will become an ootid (female gamete). Once formed lots of hormonal changes occur in female example individual's diet increase, mammary glands differentiate and all necessary changes for pregnancy which will be unnecessary at end of every month. 
I hope u understood and comment if u want to ask about above imaginary paragraph.