1)It is difficult (or, more to the point, expensive) to build physical carrier-sensing hardware(802.11 radios) for RF-based media, because transceivers can transmit and receive simultaneously only if they incorporate expensive electronics. I mean the radios with those capabilities are very expensive.
2) Beacuse of hidden node probem
Laptop-A - AP - Laptop-B
In above scenario medium between Laptop A and Access Point will be busy when they are communicatiing but Laptop B might not be aware of such a scenario. Thus, it becomes difficult to detect that medium is busy.
In wired networks the whole medium goes high. in the form of Voltage in wire rising to peek high or peek low.
Thus they use 4 defense tech. to avoid collision
1) NAV (Network allocation vector) using RTS/CTS- also called Virtual carrier sensing
2) IFS (Interframe Spacing)
3) Random Backoff
4) Physical carrier sensing (CS of CSMA)
How 4 defense tech work in tandom: