This is the top (but not only) reason keeping your baby/toddler in your lap instead of in their own seat, buckled into a carseat, is a terrible way to fly. Lap infants are far more likely to be injured in-flight than other children:
https://journals.lww.com/pec-online/fulltext/2019/10000/in_f...The carriers don't make this easy: they have conflicting rules on carseats for in-cabin use, especially if you're flying between the US and Europe, both of which have stringent carseat regulations, but absolutely no models that are approved for use in both. In practice, United allowed the EU carseat (used, bought specifically for the trip) and Lufthansa allowed a US carseat (more compact, bought for car use in the US), but both of their sites stated that they only allowed carseated approved by the airlines' local authorities: NHTSA (United) or TÜV (Lufthansa).