why the power absorbed by RL load in half wave rectifier is I^2R not V^2/R nor VI
\$V^2/R\$ is also correct if you use RMS volts.

To understand why V*I is not correct, it’s good to understand how average power of a periodic signal is calculated. 

The instantaneous power is always v(t)*i(t). 

To determine the average power you integrate it over some interval and divide by the time for the average. If the signal is periodic you can take that integral over a single cycle and you know that each cycle is the same in steady-state. 

If the load happens to be a fixed resistance, you can substitute for one of i(t) or v(t) and get, say, the integral of \$v(t)^2\$/R. We call the square root of that integral (divided by the interval time) the RMS voltage. The resistance is fixed so it can be applied later, for any resistance. 

There is no physical meaning to RMS volts * RMS amperes.

You should also note that the usefulness of RMS current or voltage is more-or-less limited to when you have resistive loads- for example it does not help much if the load is a diode.