There's a difference between "expand a little", which they were indeed doing (Brazza, Faidherbe...) and "let's paint the map Blue, white and Red!" which was OTL.Is not ego, everyone at this point knows the prestige of being a global power and the fact that they can only beat England by expanding oversea, given that if the go Napoleon they would lose any ally and get coalitioned again.
Plus everyone knows that is important to own global resources, is not useless sand, Algeria and Tunisia would important extension of France.
Also its antithetical to the behavior of most empires, France is strong and can flex his muscles by geopolitically expanding in Africa were given 2 other competitors are mostly removed, are limited to Britain and Portugal´s old holdings. Given they would have an existential threat if Britain takes all of their potential colonies, there is no reason to not expand, at least a little.
The Brits weren't THAT present before 1870. It was French moves that pushed them to conquer what was left. Same for the Portuguese as they only had coastal holdings with a few mestiso farmers.
Sure they CAN expand in Africa, but without the 1871 traumatism, they don't want to. From the French colonial point of view, it's a very expensive codpiece with some vague rationalisation about civilization and what-not
I would say that was post-facto rationalisation. Like when you splurge hundreds on new clothes "that you'll totaly wear all the time, I swear!". If that came up in the 1880's, the machine was well under wayIt was argued by some at the time that countries should try to conquer whatever lands they could, as a sort of investment in the future; a territory that might be worthless in 1880 might turn out to be valuable 50 years later. In a few cases this turned out to be true. The Algerian desert turned out to have considerable oil and gas reserves, for example.
The initial motivations were clearly French prestige and renewal of French blood (although the last bit might just be a bias in my sources, I do love my Lyautey)