Most Bluetooth headsets will connect to multiple sources at once. For example, I have some Bose headphones that will connect to two different sources and, if they are in the same room, it will connect to both at the same time. Of course, my Bose (from what I recall) can only remember those two sources -- if I add a third it won't remember the first. As you point out, the advantage is that whenever you use that device you get sound from the device when your headphones are turned on. The disadvantage, if for some reason both are in use and playing at the same time, then I get the sound from both in my headphones at the same time.
Apple, from what I can tell, took a different approach. Since they will "automatically" pair with almost any Apple device on your account, they made it so they only actually "pair" with one device at a time. So, if I use my iPad then my iPhone is typically in the room; and if I last used them on the iPhone, I have to let the AirPods know that I now want to use them on the iPad. The advantage, if someone else is using my iPad while I use my iPhone, I don't hear both audio sources. And, as you point out, the disadvantage is if I am using the iPad and get a phone call, I have to "tell" the AirPods to switch to my iPhone.
Apple, for whatever reason, appears to have decided to go with pairing a single device. I'm not sure if this was a hardware limitation, such as the antenna being so small couldn't pair with two devices at once; a practical consideration, such as pairing with two devices drains the battery faster; or just a design decision.
I personally don't mind it. As others have stated, if I want to use it with my iPad I can go to the control panel, or I can use the Audio button on the phone during a phone call, though typically when I'm pulling my AirPods out, I just put the open case next to the device I'm going to use (I often put it next to the screen) -- and this causes the AirPods to automatically figure out that is the device I want to use the AirPods with.
I accepted the "limitation" of the AirPods and don't have an issue. If you do, then I'll agree that AirPods are not for you.