No the tech companies experimented by looking at Siri, significantly improving upon what Siri can do and then bundling it up in little boxes much cheaper than iPhones with speakers much better than iPhones and seeing if they could sell lots of them. They did. They're a relative hit in spite of many people having iPhones, iPads, etc. In fact, if you read the threads even on this Apple-centric site, you'll see many of your fellow Apple fans proudly claiming to own 1+ Echos too... even though they have an iPhone, iPad, etc.
Conceptually, you are right- iPhone can pretty much do everything these Echos do. But the same can be said for iPhone vs. Apple Watch, iPhone vs. iPad. iPhone vs.

TV and maybe even iPhone vs. Macs or PC. Do you have any of those other Apple products too? Well why? Your iPhone can mostly do what they do. As soon as you start rationalizing owning a Watch or iPad or

TV or Mac too, you start making the case for how other tech can be a "want it or need it too"... like one of these things... in spite of your iPhone being able to do most or up to everything they can do too. If you can't do that for a non-Apple product, just stand by until Apple rolls out their cut at one of these things... and then magically you'll probably start seeing a fit even for your own situation.
If you don't see a potential use of one of these things in your own life, that's fine. Don't buy. But just because you think your iPhone covers all such wants for you doesn't mean everyone else feels the same. Many other people- including many people on this very site- are happy with their purchases of Echo-type devices... including many with iPhones. Are they all wrong?
Putting aside the cohabitation scenario DOES undermine the generally "stationary" product concept. If you are not single living alone, when you leave your home and take your phone, that's comparable in this example to taking an Echo from the other people back home. They want to get Alexa to do something but Alexa is out with you. Your reply implies you might have an

TV, a generally stationary device from Apple. If you are not single living alone, do others like to use that when you are out with your iPhone? If so, they can because it is generally there at home for them... unlike your iPhone which can do most of what of that

TV can do. That's the point of the stationary vs. mobile concept.
If you ARE single living alone, stationary vs. mobile doesn't matter to you... because the only ears or eyes to use any of this stuff are always wherever you happen to be. Thus whether any of that is tagged mobile or stationary is irrelevant to you. However, there are many, many people that are not single living alone. And the others in their home might want to use Echos or Apple TVs or Macs when 1+ of their family is out and about with an iPhone that can do most of the same functions.
Or maybe they want to voice command & listen to something with better speakers than iPhone.
Or maybe they want to voice command something that is much smarter than Siri.
Or maybe they want a better-than-Siri + speaker experience without paying $600-$800 more for an iPhone?
Etc.