Why use both packet detection and symbol timing offset in WLAN 802.11?
However, if we can detect the start of the packets, we should be able to find the start of the data symbol since the length of the preamble is known at the receiver, so why would we use still employ the symbol timing estimation algorithm to find the start of the data symbol.

The start of packet detection isn't accurate enough. When you look into the properties of OFDM symbol timing recovery methods. you'll notice that they use mathematical tricks to achieve higher timing accuracy.
The main function of the packet detection is to know that you need to synchronize, not to synchronize. (and of course, to avoid collisions)