Hmmm... I don't know if you realise this or not, but you're asking more than one question here.
1) Why do you create your own settings?
2) Why do you use your own homebrew settings rather than using a published one?
3) Am I still homebrewing if I'm borrowing elements from other settings?
4) If your homebrew setting is so vanilla that it's practically indistinguisable from hoardes of other such settings, then why bother creating it?
My answers:
1) Why do you create your own settings?
Cause I like the activity of creating settings. All by itself, it is a fun hobby. Some of my setting will most likely never be played in, which does sadden me somewhat, but doesn't deter me in the least from still creating settings. Usually, I don't make vanilla settings. My settings are usually exploring an idea or a set of them - a grand leap off of a "what if..?" question. Example, "What if there were no land, and all the races were either aquatic or airborne?" Then the creative juices start flowing and I'm thinking of sea creatures, subaquatic weaponery, water combat feats, etc. Then I'll spark off into imagining floating forests held aloft by air bladders, and then animals which live there and races which evolved there. Then, I'm exploring how the water and air ecologies interact. What cultures arise? What unique adventuring opportunities could exist? and so on.... It would be great if people also played in my settings (and sometimes they do), but it isn't a requirement.
2) Why do you use your own homebrew settings rather than using a published one?
Your typical well-developed setting (whether published or free) has a lot of material in it. To properly GM such a setting, I would need to read, study, and then memorise the bulk of it so that I then know enough about what that world is like so that I can run adventures in it which aren't mostly ignoring the setting and using the map only. I don't have time to do that. Also, all the fun bits are already completed. The setting is completed. Sure, I could customize it, but at that point I may as well make my own setting rather than try and force an existing one to meet my preferences.
3) Am I still homebrewing if I'm borrowing elements from other settings?
Well... That depends. Are you taking more and more elements from a single other setting such that essentially the only differences are things like the city names or the map? Or are you borrowing elements from a variety of different settings and combining them together with unique elements and creating a new setting? If you're doing the first, then perhaps you're right, you may as well just use that setting rather than filling off the serial numbers and re-creating it bit by bit.
If however you are doing the second, then you're not really copying a setting, you're creating a new one. If you take Driders from setting "X" and turn them into a benelovent forest PC race, then you pit them in a war with the cool Beholder empire from setting "Y", and you borrowed that neat Dwarven god from setting "Z" and gave it to your Dwarves, then you have created a combination of circumstances that the creators of settings X, Y, and Z never imagined and thus your setting is different and unique from all of theirs, despite the borrowing from them.
The difference between inspiration and plagerism is a combination of how much you take wholesale from elsewhere, how much you yourself contribute to the finished setting, and how you use or re-envision the elements you borrow. Doing almost nothing and just re-labeling others' efforts as your own is plagerism. Taking ideas from others, combining them with your own, and presenting them in new and interesting ways is inspiration - there's nothing wrong with that. Artists of all kinds have been influenced by prior works for tens of thousands of years.
4) If your homebrew setting is so vanilla that it's practically indistinguisable from hoardes of other such settings, then why bother creating it?
Ah... but the devil is in the details! Let's suppose that I want to play in a vanilla setting, which I occasionally do want to do. Okay, I could pick up "The Lands of Generica" and play there. But then, I see that in that setting Dwarves all live in this single mountain range and kill Goblins, Orcs, and Half-Orcs on sight. Then I notice that the Kingdom of Sunderia is ruled by a hidden council and is waging a civil war with it's theives guild. Then, I note that the King's Guard of the Kingdom of Morenda are wandering the world searching for the three lost Morendan Princes, stolen as babes 20 years ago. Anyone playing the Generica prestige class "Morendan Guard" are all wanderers, sworn to never return to their homeland until the three princes are found. Then, I read that Sorcerors in Generica use an alternate set of rules due to their ancestors' pacts with demons during the great Demon-Titan wars millenia ago. Oh, and the chapter on the Gods includes volumes of info as well.
It's all very nice, but it is at this point I start feeling stifled by all of the setting info, none of which I had a hand in creating, all of which is suggesting adventures and campaign themes which I may or may not have wanted to persue. I then find myself starting to make MY version of a generic setting which suits the setting ideas and campaign plotlines that I want to run. Even the most generic setting has it's own character and implied themes and plotlines. Does every 1st level fighter look and act the same? Nope, and neither does every FR knockoff.