Because a state funeral is for the Monarch. On the very rare occasions they are granted by special royal privalege to a person whom the country deems requires a special funeral. Such people such as Princess Diana, The Queen Mother, Churchill etc were all considered special enough that the country almost demanded that the death of those people should be recognised by a grand state occasion such as a state funeral.
An ordinary sailor/Soldier/airman does not warrant that.
The comment about the RN referred to the fact that the RN lead the procession on a state funeral with ratings pulling the funeral carriage. Not sure we have the manpower available to do that anymore.