Smart home devices speak several different languages. Some use Wi-Fi directly, others use low-power mesh protocols that need a hub, and newer devices use a shared standard designed to bridge them.
That is why two devices can sit on the same shelf and be invisible to each other: without a hub or bridge that speaks both, there is no common path.
Ecosystem lock-in adds a second layer. A device can be technically compatible yet unavailable in a particular assistant's app because the manufacturer never published the integration.
The practical strategy is to choose one primary ecosystem, check compatibility before buying, and prefer devices supporting the shared standard when the choice is close.