All of the above. If the coffee pot and your laptop have to be secured during take off and landing (when there’s the greatest chance of an incident- aborted takeoff, aborted landing, blown tire, etc.), shouldn’t your baby? Also, plane seat belts don’t fit until roughly 40 lbs, which is roughly a 4-5 year old. Before that, the belt doesn’t fit properly and the child could submarine underneath it in the event of an incident. There’s also the maturity factor- most younger kids are going to quickly figure out how to unbuckle a plane belt and are going to want to be up and down, walking around. If they do that during preparation for takeoff and you can’t get them to stay buckled in their seat when the seat belt sign is on, you can be kicked off the plane as it’s a matter of safety.
Kids are used to sitting in their car seats and it’s the safest way to keep them contained. It’s a pain lugging car seats through the airport (been there, done that with 2 seats at a time) but it’s not safe to check seats (gate or with luggage) and is the safest (and most comfortable) way for kids to travel on planes.
Here’s a good link that goes into more depth on safe plane travel with littles-
https://csftl.org/leaving-on-a-jet-plane-the-csftl-guide-to-safe-air-travel-with-children/ .