Why does a large industrial sector not always indicate that a nation is fully developed?

Because having a large industrial sector tells us more or less only that - this country have a large industrial sector. While additional inferences can be made, it's very hard to make big assumptons about different aspects of a country's economy. We don't now whether they're a major exporter of some goods and is making enough money to be able to suffice all the criteria we have for a developed nation (not that making a lot of money is the only aspect here, there are many others). 

His ballance should be $65