One must not only consider short-term benefits but also long-term interests, after all, the construction of the Floating City is not something that could be achieved in the short term, one cannot engage in one-off transactions, as harvesting leeks can't be done root and stem, otherwise, while there might be short-term profits, the losses would definitely be even greater.
Talking big is just speech, but actually implementing means to surrender.
As for why Qin Muye didn't follow along?
As for making money, Qin Muye had his methods. In the current situation, arms dealing was definitely the most profitable, as for food and such, that was not profitable at all. The Magic Continent wasn't a Transcendent magicless world after all; any Druid or Mage could easily create fertilizers akin to chemical fertilizers.