Possibly a feature of the single-axle 737 undercarriage
Yes, I agree, but I notice the A320 series don't arrive with such a thump. I would have thought it was important to design in a softer undercarriage, and not just for softy passenger reasons. It must stress the fuselage thumping in 40-50,000 landings over an airframe lifetime. You can see the effects in the fuselage aft of the wing- wrinkling developing. Then you get more of the 'Indonesian' type landings where the fuselage collapses altogether either side of the wing- something I don't think an A320 series has experienced at all, but several 737s are photographed in that state!