Why can you use the being verb twice in a past and present tense sentences and not in the future tense sentences?
Past: I was being silly.
Present: I am being silly.
Future: I will be silly. not I will be being silly?
It is possible to use that construction, e.g.
1. If I say that, I will be being very silly.
However, it's not very often used. I think this is because progressive constructions place us vividly in the middle of the action or state; which is more difficult in the context of an action or state that hasn't yet happened, than in a past or present action or state.
That said, other members may have other views!
All the best,
I think we don't say it because the be being sound is awkward to say.
Try saying it aloud, you'll see what I mean.