It's interesting that while many fails, there is still one that wins. That is if you combine the efforts of these 4 digital assistants, you'll get a much smarter one. Do they have an API? Can you query siri, cortana, etc..?
There are APIs for both Amazon and Google's voice assistant services. Not surprisingly Siri doesn't expose a public one, I've no idea about Cortana. I've messed around a little with them on the Raspberry Pi.
This idea, while simple in principle, might be kinda annoying in practice. You're still left with similar issues - how do you decide which talking cylinder service answered the question best? Do you play all of the answers? For me I'm fairly sure listening to all of them in a row would frustrate me even further - just waiting for Alexa to finish telling me the news headlines is sometimes kinda annoying, especially when that information in visual form can be grokked almost instantly. Many of these devices, especially the Google one, are getting better at context based followup questions - managing who to send your follow up question to could be kinda crappy as well. I suppose you could do one device that could ask each service individually ("Alexa...", "Ok Google..."), but in my experience as soon as I get one bad answer, I inevitably just use google.com to find what I need rather than risk wasting my time on another failed conversation.
The main part that I've found hard to do in home rolled voice assistants is microphone arrays. Almost all these devices use pretty sophisticated microphone technologies for things like noise cancelling, subject isolation etc, which so far has been non-trivial to do to a similar standard in homemade versions of them. It also certainly used to be the case that creating your own "hotword" system to call the Alexa API was technically against the ToS (it allowed you to use a button press to call Alexa instead), as naturally Amazon would rather you buy a real Echo. No idea if this is still the case, and at any rate Amazon can't really enforce this either, but worth mentioning.
