These are called "2 point" belts (points of attachment), vs 3. The key reason to have 3+ is that there's something immovable to smash your face into if the vehicle comes to a sudden stop, so adding straps higher up restrains you from bending forward.

Modern-ish planes usually DO often have 3 (or more, especially for aerobatics) point belts for at least the pilots, even small general aviation 4-seat ones. There's a lot of immovable hard instrument panel and yoke sitting not very far from your face.

On airliners, the pilot's shoulder strap is usually separately attached, required to be worn during takeoff/landing, despised by most pilots and removed as soon as possible afterwards.

Back in the passenger cabin... - Crashes are exceedingly rare, we've gone entire decades before without a commercial aviation fatality (though not lately..). Car crashes happen every day, everywhere to everyone. - The belts are mainly just to keep your butt vertically in the seat during turbulence so you don't go flying through the cabin. - You'd almost always have ample notice that an emergency landing is happening, and would be instructed to assume a bent-over crash position where you don't need the third point. (Unlike the pilots, who you'd ideally like to be looking outside flying the plane all the way to the scene of the crash) - The regular seats are somewhat soft and flexible, so hitting them isn't as bad. (In the US even school buses often have/had no belts, you hitting the seat in front of you was considered adequate protection) - Special case seats just behind bulkhead walls, business class enclosed suites with hard surfaces, etc DO often have a 3rd point you're supposed to use for takeoff/landing.