This is because reactive power concept is completely different from active(real) power,

That is when we talk about active (real) power, we deal with two terms :

The average value, and the instantaneous value.

Both has significant values; let's say the average value is 5kw, where the instantaneous value maybe 2kw or 6kw, etc...

On the otherhand, reactive power has always a Zero average value, and a different significant instantaneous values, but since the instantaneous value is difficult to measure, we always take the maximum instantaneous value of reactive power and deal with it as a textbook terminology (Reactive power) which is symbolized as Q and equals V I sintheta.

And for the same reason (difficult to measure instantaneous value ) we easily deal with the average value of the active power other than the instantaneous value.

As a brief:

(Active power) symbolized P or known sometimes as real power equals V I cos theta

represents the average value.

(Reactive power) symbolized Q equals V I sin theta represents maximum instantaneous value.

Someone can ask!

Why not to take the maximum instantaneous value of average power such as reactive power as a terminology?

Easily , why not to unites the two concepts ?!

This is the answer of our main question (Why reactive power is so confusing for engineers?)

This is easily because ,there is no useful meaning of using the instantaneous value for the active power, because it is a real produced power goes in one direction from source to load, where the average reactive power is always zero valued, since it goes back and forth in the network, and we are forced to deal with it in some way as an indication value and give it a unique terminology to express it as a fact that we cannot skip.

In other words, you cannot compare two different things; apple and banana! Each one has a different taste.

I hope that I answered the question, and dislodged some dust about this confusing concept, even I didn't take the boring mathematical path.