In article .com,
"Robert M. Gary" wrote:

The "no seat" rule is just there to make the airlines happy because
they can't sell a full fare ticket for an infant (they can try but
won't get a lot of takers).

It's a little more sophisticated than that.
Airline seat backs are not as rigid as auto seats. The problem with the
earlier safety seats was that they would not withstand a rear impact and
would fold in half, crushing the child.
Newer, approved child safety seats are supposed to be more resistant to
folding.