Flight Attendants usually sit in a backwards facing way, which if the plane would come to some sudden stop, would throw them usually backwards to I think, and they are needed for safety too, so they need the protection
With a car, there are so many potential ways to get in a crash, like getting struck from the side, front, back, etc. In a plane, the main reason a seatbelt exists is for turbulence and takeoff and landing, so basically keeping you in your seat. Thus, a shoulder strap would be overkill. If you look at a lot of baby carriers, you’ll notice the same thing.
This might not be true but i would assume the shoulder straps would help flight attendants because they don’t have anything to hold on to during turbulence like arm rests.
It’s really a big combination of smaller reasons at which this is the case.
For one, the reasons of putting a seatbelt in a car is not the same as that on an aircraft. In a car they’re designed to avoid you from fatally colliding with what is in front of you (they’re paired with airbags nowadays, so you can understand the gist of what they’re preventing). In a large, high speed commercial airliners crash, it’s most likely that it is probably too late, and the sheer force will negate whatever type of seatbelt you add. At that point, the better option at that point is to not get an aircraft in that situation in the first place; that’s why checks, walkarounds and maintenance routines are that thorough.
Combine this with elements of passenger comfort, emergency practicality and whatnot, it’s just not going to be a benefit in the end.
A single over-the-lap belt would do more than enough. An aircraft of that sort shouldn’t even be in those conditions in the first place to the point that it warrants the need for a shoulder strap.
I’ll also add… in most cases, conditions in which would require a shoulder strap as it would in a car are usually much more predictable in a plane giving the passengers time to actually brace themselves. In a car, this time is just non existent because the crash would most likely occur in a matter of milliseconds. For example, if a plane needed to ditch into a body of water which would cause a rapid deceleration in the plane, passengers would be instructed to brace which would prevent any kind of frontal impact that a over the shoulder seatbelt would prevent in a car.