Second Order Passive RC Low Pass Filter Doesn't Have -6dB at cutoff Frequency in LTSpice
Why is it so? What am I doing wrong?

I would like you to explain where you got the formula: 

$$fc = \frac{1}{2\pi \sqrt {R_1C_1R_2C_2}}$$

That formula applies to a Sallen-Key second order filter but that's not what you made. You simply cascaded (connected one after the other) two first order RC lowpass filters. The way you did that (with no buffering in between) means that you cannot simply apply the formulas for a first order filter and square it.

the gain at cutoff frequency comes out to be -19.08dB

You cannot define the cutoff frequency like that, for a low pass filter the gain at the cutoff point is by definiton -3 dB. So you find the -3 dB gain point and the corresponding frequency, that frequency is the cutoff point. That also means that if you cascade two first order lowpass filters (with a buffer in between so the transfer functions can simply be multiplied) that the cutoff point will move to a lower frequency.