Why does OSPF use flooding instead of broadcasting? In flooding, we send unicast/multicast frames to all the routers in the network, whereas in broadcasting, we have a broadcast address by which we send packets to all the routers. In OSPF, "Router disseminates LSPs (link state packets) to all the routers excluding itself and not only to its neighbors." So why can't we use directed or limited broadcasting here to send these LSPs to all routers? Why is flooding used here instead of broadcasting?

You are confusing the layer-2 flooding with OSPF flooding. The routers in an OSPF area may not all be directly connected, but every router in the area needs to have a full understanding of all the other routers and networks in the area. OSPF floods this information to every other router in the area, even if some of the other routers may not be reachable by layer-2 frames from the flooding router.

Broadcast requires that the routers be on the same network. This is strictly true for Limited Broadcast, and Directed Broadcast should be disabled due to security concerns (see _RFC 2644, Changing the Default for Directed Broadcasts in Routers_).