I worked for a US carrier for 6 years. 3 years as a ticket counter/gate agent and supervisor, 3 years as a flight attendant and trainer.
The reason you have to pay extra if your bag is overweight is because heavy bags shift around in the aircraft bag bin, often causing damage to the aircraft and other people's luggage. The airline has to pay to fix both. Not to mention the injuries lifting overweight bags causes...ouch...happened to me too...
Most aircraft is leased by a carrier, not owned. If an airline was so swap out all their seating on their whole fleet can you imagine how expensive that would be? And who do you think that cost would be passed down to? There are larger seats available for purchase, they're called business and first class. Unfortunately us consumers (including myself) want low airfares. In order to keep the fares low but still make a profit the airlines have to cut corners and service wherever they can.
The airline I worked for had a policy that if a passenger was too large to lower the armrest next to them and/or to big to use a seatbelt even with a seatbelt extension they needed two seats. If there was an extra seat on the plane we would switch people around so the "passenger of size" could have two seats next to each other. At no extra cost. If there weren't any available seats we would have to ask them to get off and rebook them on another flight. It's not just an issue of comfort for their fellow passengers, it's an issue of safety.
When you make an airline reservation you know how big you are and how much room you need. You have the option of purchasing seats. It's not fair to make other passengers suffer because of your lifestyle choices.
As far as reclining seats far back, it's very inconsiderate. I never had a problem nicely asking telling someone not to recline so far back because the person behind them was very uncomfortable.
The reason very small planes (well not the plane, the person doing weight and balance) need to know your weight is because it makes a huge difference. They have to balance out the weight of the plane, passengers, cargo, and fuel and it's a big pain in the neck!
Larger carriers couldn't charge ticket prices according to each passenger's weight. Everyone would lie (well I would since I'm vain and lie on my driver's license) and the ticket agent would have to weigh each passenger then charge them extra. Boarding would take about 3 hours...