As someone that works in this arena, do you know why they do that? It's because each seat/passenger(souls on board) is rated at 200lbs/person. This is how they calculate the proper weight and balance of the plane in order to set the right takeoff and landing weights and that they know they'll be able to take off. The weight distribution on a larger plane is not as critical as smaller ones, but the TOTAL weight of the plane (to include fuel and baggage) is still critically important. Further, the other reason planes (pre-covid) are always overbooked is that the profit margin, even on a completely full flight is razor thin. Back in the mid 2000's when oil prices spiked, some carries were only profiting about $100 per flight (full flight). That's when you saw us all getting bag fees as well, to offset the losses they were taking across the industry.
If someone is 300 lbs, they should be charged a surcharge on the extra weight.
Ever seen the result of a load shift on an aircraft? woof. Granted this isn't going to happen on a passenger flight, but outlines how important having your weights right at takeoff...