Why is 1-bromo-3-methylbut-2-ene named like that and not as 2-(bromomethyl)but-2-ene?
Here's 1-bromo-3-methylbut-2-ene's structure:



You have only two possible chains to choose from: a 3-carbon chain and a 4-carbon chain (doesn't matter which methyl group would be part of that chain, the other one will become a substituent on the chain). 

You won't choose the 3-C chain for two reasons: 


It is not the longest chain.
It does not contain the double bond in the molecule. 


Thus, you are left with a 4-C chain, which necessarily includes the bromine atom.