Qualitative adverbs originating from possessive adjectives in sg. inst. come in two varieties in Russian:
The adjective they modify describes some quality pertinent to the noun from which the adverb is formed:
The adjective they modify describes some quality similar to the noun from which the adverb is formed. They usually come with the preposition по-:
- Он по-гусарски расточителен // He's as prodigal with money as a hussar (lit. "hussarly prodigal")
- Макароны по-флотски // Pasta a la navy, a dish similar to ragu bolognese (lit. "navally (cooked) pasta, pasta as they do in the navy")
If you look closely, there is a difference between those adjectives.
The ones from the first list are about the thing the adverb describes: ideology is something which is good, ballistic is something the fabric is proof against etc.
The ones from the second list are like the thing the adverb describes: he's prodigal like a hussar, the pasta is like they do in the navy etc.
However, this is quite a fine line, and some of the adjectives are bleeding from one category to another. There is a number of adjectives like героически, чертовски, ангельски, дьявольски, творчески etc. which should have had the по- before them but they don't:
- Он дьявольски умён // He's smart as a devil
- Защитники Москвы героически пали на поле боя // The defenders of Moscow had fallen on the battlefield like heroes
In both those cases, по- would have had more sense.
Житейски in your example belongs to the first category: his everyday skills are what he is inexperienced at, not like something he's inexperienced at. Taken literally, он по-житейски неопытен would mean something like "he's inexperienced the way those who live their daily lives are", which makes little sense.
However, as I said, this is quite a fine line, and по-житейски and житейски are sometimes mixed:
Смагин был обеспечен, перспективен и совершенно по-житейски неопытен.
Губернатор Краснодарского края Александр Ткачёв заявил, что станица Кущёвская – это "по-житейски благополучное место"