I'll bet that is less of an issue than the issue of evacuation.
Currently, aircraft seatbelts are designed for very quick release when needed with no training or muscle memory needed, shove your hand at your waist and pull the metal flap and it immediately comes apart. This seems like it could be an issue with baby carriers, since a) you can probably expect a bit more complex arrangement of straps buckles and b) lack of muscle memory or training on presumably a different style of carrier than what you ever ever use outside an airplane.
A second issue that comes to mind, how to do seating arrangements? Imagine a parent struggling to remove baby during a emergency. Imagine a 3 seat row window side, where others are stuck waiting for parent to release baby.
This seems like a certification nightmare
You are only allowed to have the carrier in the window seat from the flights I’ve been on.
Certified rigid car seats are already allowed onboard and pretty much every airline you've ever flown on has clear guidelines on which seats they can be placed in for exactly that reason.