Why calculated fields or Business Rules cannot use lookup fields?

So i have a lookup field in parent entity and am trying to update the children with the amount from that lookup field, but it will not copy the text into the child's field unless it's also a lookup field.

That is fine so far. 

I go to account and i copy account owing balance into contact.

Now in Contact Record i need that amount (lookup field) to be used in a calculation for another field. However, I can't use the contents of that lookup field in my calculations. Created a dummy field to copy the contents of lookup field into that and use that, it doesn't work. 

Anyone has a suggestion?

thank you

E