Mr Boeing builds a wing, or his contractors do, which is designed to be efficient at high-ish Mach no's.
It must be, otherwise it wouldn't be economical to fly the thing.
Bring it close to thr ground,and therrfore slow, it is outside its design parameters; so it needs high lift devices. Slats, LE flaps,TE flaps; ergo, it is not at its design efficiency.
Bolt on a pair of underslung engines and the thrust couples and you have a delicate balance.
So it lands firmly; gravity always wins.