why won't my leaderstats save?

Wiki's definition on PlayerRemoving:

The PlayerRemoving event fires right before a Player is leaves the game. This event fires before Instance/ChildRemoved|ChildRemoved does on Players, and behaves somewhat similarly to Instance/DescendantRemoving. Since event fires before the actual removal of a Player, this event is useful to store player data using a GlobalDataStore.

Meaning the data wasn't saving because the player had already left. Simply switch the events, and referring back to my comments, please indent your code properly and always use local variables.

local cashSave = game:GetService("DataStoreService"):GetDataStore("SaveCash") -- No need to declare this variable each time a player joins. 
local Players = game:GetService("Players")
local reward = 1 -- Same here.

Players.PlayerAdded:Connect(function(player)
    local leaderstats = Instance.new("IntValue")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = player

    local cash = Instance.new("IntValue")
    cash.Name = "RPCash"
    cash.Value = cashSave:GetAsync(player.UserId)
    cash.Parent = leaderstats
end)

Players.PlayerRemoving:Connect(function(player)
    cashSave:UpdateAsync(player.UserId, function(previous)
        -- You're simply updating the data store, use this instead
        -- 'previous' is just the old data

        return player.leaderstats.RPCash.Value
        -- function must return what you want to update with
    end)
end)

while true do
    -- much better than having individual loops 
    wait(5) 
    for _, player in ipairs(Players:GetPlayers()) do
        player.leaderstats.RPCash.Value = player.leaderstats.RPCash.Value + reward
    end
end

Here is some information on :UpdateAsync(), and here for a tutorial written by Link150 on how to properly save your player's data.