You can, however all these alternatives are measurably worse in terms of features and ease-of-use, so much so that switching to them is both painful at first and frustrating on a daily basis. Consider the sheer number of people who in every DDG thread keep on saying "well yes DDG results are worse than Google's, but you can use Google from DDG so it's okay". Well it's not okay for most people.
Also, bangs seem like a great idea at first, until you realise that Google simply does not need them to return relevant results! Then they suddenly seem way less differenciating and more like an acknowledgement of the inferiority of DDG compared to Google. And as a user, I'd much rather type "duckduckgo" and get the official site at first then the wikipedia entry (with a complimentary knowledge box on the side) than have to decide first where I want to look and then type "!w duckduckgo".
Even in this very thread, we can read stuff like
> "I've been using DuckDuckGo for years now. It's not perfect but if you think carefully about your query it will work fine.
> For me the killer feature is DDG's bang commands (https://duckduckgo.com/bang) prepend "!w" for wikipedia, "!imdb" for movie trivia stuff, etc."
Well, Google will achieve the same or better results without this need to mentally switch context. This is a feature, and a key differentiator.