First of all, it's a lot more than some. And contrary to what you are saying, Scotland is not poor with a small undiversified economy. What utter nonsense. But let me see, why do they want independence? To be a normal country for starters. It's about much, much, much, much more than economics.
The overwhelming majority of countries are independent and run their own affairs and that is something that many Scots reckon is worth aspiring to.
A political union which is so imbalanced to the extent that one of the parties is outnumbered 10 to 1 is not equality. I reckon if you were to ask citizens of Austria, Czech Republic, Belgium, Iceland, Switzerland, Andorra, Norway, Sweden, Finland, Denmark, Slovenia, Croatia, Isle of Man, Estonia, Latvia, Lithuania, Bosnia & Herzegovina, Montenegro, Liechtenstein, Macedonia, Poland, Slovakia, Malta, Hungary, Greece, Moldova, Portugal, Ireland, for example if they'd like to give up their independence and enter into a political union where it didn't matter how they voted in an election, it would have no bearing whatsoever on who they would receive as the government, it's highly unlikely they'd say "Oh goody, yes please."