Jehovah's Witnesses, since we don't know the exact vowels of YHWH, why not call God Our Father?

Since YHWH could actually be Jehovah, Joheeviha, Jahiviaah or any other possibility of the 5 vowels, it is not just a matter of pronunciation -- it's a matter of being way way off and calling Him a Name that was never really His Holy Name. It seems reasonable to simply call Him Our Heavenly Father, Father God since we are His children or Lord. Why not? Jesus said to "Pray in this way" -- Our Father who art in Heaven Again, if we actually knew the Name, it would make sense to use it all of the time -- but the fact is - we DON'T know it but have substituted vowels from Adonai.