Because it’s a huge source of revenue:
[F]or a long time, the fee [paid by parents for extra-legal births] has been collected and spent in opacity, without even a hint as to how revenues generated from it are allocated in most local governments’ annual budgetary reports.
On July 11, 2013, Youshui Wu, a lawyer from Zhejiang Province, tackled this issue by submitting applications to the family planning commissions and treasury bureaus of 31 provincial-level political authorities, requesting the reports of how the money was levied and spent in 2012. By August 31, 17 provinces had responded with information on the amount of money collected in social support fees in 2012, yet none explained how the money was spent. The fees levied in the 17 provinces totaled 16.5 billion RMB (about $2.7 billion US dollars). …
Within this context, it is easier to understand why, despite strong public objection to the policy and academic proof of the policy’s long-term harm on China’s demographic structure, the strict birth control policy has remained resistant to reform.