They exist, e.g.the CARE harness..
Problem here is do you want the airline to supply them/cart an unknown number around on every flight, just in case, or should parents of children likely to benefit from them go out and buy something suitable?
FWIW I flew airliners around for 30 plus years, have seen/heard about the odd small drama with toddlers but it was always resolved with nil or minimal delay..
The Porter/Victoria cancellation is definitely an outlier and so I can't see there's a financial case for the industry to be buying and carrying toddler harnesses in bulk.