Explain why you cannot directly connect a high-pass and low-pass filter to form a band-pass filter with a transfer function equal to the product of the high-pass and low-pass transfer functions

The main issue associated with band pass filter created by combining directly a high-pass and a low-pass filter is that the gain is always less than unity and this makes the load impedance to have impact on the filters characteristics

Explanation:

Low-Pass Filters: filter out all frequency above the cut-off frequency and passes all frequencies below the cut-off frequency.

High-Pass Filters: likewise, high-pass filter attenuate all frequency below the cut-off frequency and passes all frequencies above the cut-off frequency.

Band-pass filter : passes frequency between a range, and attenuate all frequencies outside that particular range.

Combining directly a high-pass and low-pass filter forms a passive band-pass filter, the main demerit of passive filters is that the output signal's amplitude is usually less than that of the input signal. Hence, their gain is less than unity.