1. You may already have a brand, but many don't - so they want people to switch.
2. People who don't drink soda need to get hooked somehow!
3. Increasing consumption. Think about the "Got Milk?" campaigns of the 90s - they weren't saying "drink XYZ brand milk", they were looking to increase milk consumption.
I think #3 is probably the biggest factor, tbh. That's why you see ads for cotton, milk, beef, avocado, etc.
#3 is because there are middlemen that don't count. The middlemen in dairy want you to buy their brand of milk. However the guy who owns the cows sells to whichever dairy gives the best deal (the big farms all have a contract with one dairy to supply only them, but there are enough small guys who sell to who ever they can). The guy who owns the cows wants you to buy milk and doesn't care what brand it is.
The same applies to the others you named: sure there are brand, but there is something large below the brand that doesn't care what brand, only that you are eating.
I think you're missing the biggest reason for product advertising, which is to notify/remind potential users/consumers that the product exists.
Many people are unaware that a given product exists at all, and others are unaware of all its potential uses; even occasional users can sometimes forget that they liked a product or found it useful.
2. People who don't drink soda need to get hooked somehow!
3. Increasing consumption. Think about the "Got Milk?" campaigns of the 90s - they weren't saying "drink XYZ brand milk", they were looking to increase milk consumption.
I think #3 is probably the biggest factor, tbh. That's why you see ads for cotton, milk, beef, avocado, etc.