Why are 2x4's actually 1.5x3.5?

When I tear into an older home doing remodeling generally built in the 60’s and earlier they have true 1×4’s, 2×4’s 2×6’s, 2×8’s etc. Nowadays all the lumber you get is still called a 1×4, 2×4, 2×8 etc but they aren’t really that size. A 1×4 is actually .75 inches by 3.5 inches. A 2×4 is actually 1.5 inches x 3.5 inches. 2×6’s are 1.5 inches x 5.5 inches. When you get up to 1×8, 2×8, 2×10,2×12 you lose another quarter of an inch. A 2×8 is 1.5inches x 7.25 inches. A 2×10 is 1.5 inches x 9.25 inches. A 2×12 is 1.5 inches by 11.25. Why did the size change?? As I mentioned before this happened in the 60’s. They used to be made of oak commonly also which is very hard and can be hard to drive nails into. Now they are almost all some type of spruce or pine. Did we run out of oak or did people get tired of bending nails over and switch to pine and spruce?