Why are recommended oil weights lower for many newer cars?

about right but the main reason for thinner oils is that of making engines more efficient and better environment friendly. To make a 4 stroke engine "cleaner" requires the engine to run or turn over faster..to do that they have to have shorter strokes which leads to a small engine and thereby smaller oil galleries to transfer oil around the rotating parts (90% of wear is produced in the first few revolutions of the crank when starting before hydrodynamic lubrication is achieved...hence the use of low viscosity oils...the 10 in 10w/30 is the oils viscosity at 40 Deg C and the 30 is the oil viscosity at 100 Deg C....So having an oil Ow/30 means the oil will get to those rotating parts a bit faster at the lower temperature

Better engine manufacturing quality control permitted less bearing clearance. Lower viscosity oils are needed in the smaller bearing clearances to permit enough oil flow to keep bearings cool ( bushing type bronze primarily but also good for rolling element hard steel bearings). Secondary advantage; lower shear resistance in thin oils makes less resistance and higher fuel economy. Also less cranking resistance on a cold start. I remember long ago , trying to start a car in exceptionally cold Chicago weather , kept the battery warm in the apartment overnight. Next morning put it in the car ; the engine turned over about twice and stopped , after market oil pressure gauge said 30 psi oil ; 10 W30 oil.