I don't think that a tool unifying all these apps is desirable nor even possible right now (because most of the protocols used are closed anyway): You'd either end up with an atrocity that tries to accommodate each feature of each of these apps or you'd have to settle for the lowest common denominator, which probably is just video, screensharing and chat. The latter might not be the worst outcome. In fact, an app that does just that could provide a superior user experience.

This begs the question though why this isn't possible with existing solutions already. Why do collaboration tool providers seem to be motivated to make their products ever more complicated or sometimes even worse than the previous version? Feature creep is a part of that problem but often collaboration tools are only seen as a small component of a more comprehensive ecosystem, as exemplified by the Skype / Skype for Business / Lync trainwreck. Skype was (and still is for the most part) an at least adequate solution. Then Microsoft bought them, intentionally made the product worse in some respects while rebranding their less-than-adequate solution Lync as Skype for Business in order to push their other enterprise products.

Why is there - as you rightly said - no common standard like email that works across all UI implementations? I think the answer to that question predominantly lies with the fact that we still have too little remote collaboration, particularly in-between organisations. As soon as remote collaboration becomes more widespread - which it inevitably will - the default even rather than the outlier we'll sooner or later have to agree on common standards just as we had to with technologies like the phone or electric power distribution.