Hello Sathish
The reason why OSPF will not allow summarization within an area is due to the very nature of the protocol. OSPF is a link state routing protocol, and by definition, every router within the same area must build an identical link-state database (LSDB) for the SPF calculations to take place correctly. If LSAs or routes are filtered within the area, the routers are no longer guaranteed to have an identical LSDB.
I hope this has been helpful!
Laz