I'll try to reply with three shorts answers.
1) Because overall quality of a signal for directly connected stages will be very bad. A coupling network allows the following: blocking DC currents (RC coupling), keeping effective load resistance the same (transformer coupling), keeping the required bandwidth at the final stage.
2) Transformer coupling provides higher voltage gain than RC coupling, higher stages' impedance matching, direct current will not be transferred to the next stage, signal power does not decrease in the base resistors or collector.
3) Because the transformer coupling is very ineffective at low frequencies. Low frequency coupling transformers require a lot of wires and material for its magnetic core, thus making the transformers bulky, heavy and expensive.
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