The first is an old answer which is why were zigbee and zwave ever developed at all, why not use Wi-Fi from the beginning? See the following FAQ (this is a clickable link) Although Wi-Fi devices have gotten cheaper to buy since this FAQ was written, all of the other issues still apply. ( I Cross posted with @sidjohn1 's excellent post, but the issues he is describing are the ones discussed in the FAQ.)
The second is a newer, but likely more temporary answer, which is that right now smartthings offers the ability to create much more complex logic rules than Echo or HomeKit. But the Hubless systems are continually evolving and adding more capabilities, so it’s not clear how long this will be true. Or how many people really need more complex logic anyway. But for now, you can create much more complex rules and schedules with SmartThings then you can with the hubless systems.
So there will be many people for whom the hubless systems work just fine, and you’re right, they will not need to look at a system like SmartThings. Others will want more complex rules or a larger number of devices, and then they may need to go beyond the current hubless systems. There’s no one right answer: the challenge is to find a system that works well for what you need. Choice is good.
