That's from UK's perspective. From EU's perspective EU got everything they want and they made a brochure about it: https://ec.europa.eu/info/sites/info/files/eu-uk_trade_and_c...

>What is the advantage for Italy or Spain to stay in the union?

The single market and and access to trade deals EU negotiated with the rest of the world. You may say that maybe Italy and Spain should get their own trade deals instead of EU's but EU's trade agreements are theirs already.

EU is not a shop in Brussels where some foreigners produce regulations or agreements. Each country elects and appoints people who do these things.