Question to Gamemaster regarding Upgrading fighting levels for Naval Fleets

I just came across this when trying to better my naval strength but it seems that it is not possible to upgrade the fighting levels of fleets like you would be able to with other land or air units. Like if you go onto "upgrade fighting levels" in the "military units" tab of the "army" dropdown, no fleets are there to be seen?

I just wonder if that is on purpose and if so, why? And if there is no reason why fleet fighting levels shouldn't be able to be upgraded like other military units, is there any plans to remedy this issue in the future?