It's only accidentally illegal. Nobody made it illegal on purpose. It just so happened that car seat regulations and car seat testing regulations were highly prescriptive, but also written at a time where nobody had thought to build a full bench, multi-child car seat.

The full bench car seat is significantly safer than single seats. But it cannot be tested on the testing sleds in North America, and the wording of the regulations implicitly forbids multi-child seats anyway in many ways. Unfortunately, fixing the regulations is probably ~10 years and ~20 million dollars in lobbying.