Questions in regards to accounting for merchandising transactions

This are questions that I feel that the textbook didn't substantiate.

Questions:

1. Why is COGS not a contra revenue account or is not on the expense section of an income statement?
2. What's the need for having gross method and net method for merchandising?  My textbook is always mentioning that the gross method is the preferred one due to updated revenue recognition principle.
3. Why is purchase considered an expense account If you still haven't consumed the merchandise that you bought? When we buy supplies on credit or cash we never debit supplies expense because we just bought the supply and we haven't consumed it yet in our operation.