Maybe your father didn't have a will and that is why you were told nothing, or if he had one, perhaps you were not named as a beneficiary in it.
Check at the county courthouse probate court (many probate courts have records online now, and you can do a check by using his name. If he had a probate file, you can go to the courthouse to look at it. If he did have one, the executor should have notified you by mail about it.
Why would you think it unusual to have the decedent's adult children there are the reading of the will? They probably were beneficiaries and wanted to find out what they will be getting and they had a general interest in wanting to find out what the will says.