Q.Why do we avoid RC and transformer coupling for amplifying extremely low frequency signals?

At very low frequencies capacitors and transformers cannot pass the signal effectively. …

[!TLDR]

Coupling capacitors block low frequencies and transformers become impractically large — use direct coupling.

The reactance of a coupling capacitor is $X_C = 1/(2\pi f C)$, which becomes very large as the frequency $f$ approaches zero, so almost none of a very-low-frequency (or DC) signal is passed to the next stage. A coupling transformer would require a very large iron core to work at such low frequencies, making it bulky and costly, and it too cannot pass DC. Therefore RC and transformer coupling are unsuitable for extremely low frequency signals, and direct coupling (which has no series capacitor or transformer) is used. …