Excellent question, 'Real... and may you have peace!
Actually, there is a VERY simple... and TRUTHFUL... explanation for that: Jehovah's Witness cannot CALL God 'our Father', for He is NOT their father; for they are NOT 'sons'. Truthfully, as far as THEY are concerned, only about 8,000 of them are 'sons of God'. Thus, if they called Him "Father", that would mean that they, too, were 'sons'. And if they, too, were 'sons', they would be in the New Covenant. And if they were in the New Covenant... THEY WOULD HAVE TO EAT AND DRINK THE FLESH AND BLOOD OF MY LORD. Yes?
And if they called themselves 'sons', that would mean they were BEGOTTEN... and ADOPTED... by holy spirit. And THAT would make them 'anointed'. However, they do not believe that the 'great crowd' are also SONS... anointed by God's holy spirit through Christ and thus, part of 'Israel'. Although it is.
As a side point, they also call Him "Jehovah", for that is the erroneous name He has allowed them to know and use. Why? Because NO reproach can be brought upon His most 'beauteous' name... and JW's... in fact the entire WTBTS conglomerate... bring reproach all the time. Can you imagine what folks think of "Jehovah" because of them?
But, in fact and in TRUTH... my Father's name is JAH. And His Son, my Lord, is named JAH-ESHUA (JAH saves), and is the MISHCHAJAH (messiah or 'chosen of JAH'), the one who 'comes in the name of JAH.
Psalm 68:4
The term "Jehovah" is an incorrect rendering of the letters YHWH (Hebrew) or JHVH (English). Many years ago, scholars ASSUMED the name to have three syllables and put an 'e', 'o' and 'a' where there were no vowels. However, the CORRECT rendering is YaHWeH (Hebrew) or JaHVeH... BOTH of which are correctly pronounced "Jah-VEH" (in Hebrew, a "y" SOUNDS like a "v"). And the name translates into "JAH (Yah) exhales" or "JAH (Yah) breathes" and means "JAH breathes into existence" or "JAH breathes and it comes to be."
My Lord's name is pronounced in Hebrew "YahShua" or "Yah-e-Shua"... and in English "Joshua" (only j's are sound like y's and so it is 'Yoshua')
There are other names and words that contain my Father's name as well:
Elijah, which is correctly pronounced EE-lee-yah, and means God (El) is (i) JAH (jah).
Joel, which is correctly pronounced Yah-El, and means JAH... is God (El).
Hallelujah, which is correctly pronounced ah-lay-loo-yah, and means praise (the 'h' is silent and so the first part of the word is correctly pronounced 'allelu') JAH, you people. Praise JAH, you people. Allelujah... or hallelujah. Hallel... or allel... is praise... lu... is you (plural)... and Jah.
I hope this helps you understand a few things more clearly, 'Real.
A slave of Christ,
SJ