People are simply willing to pay a whole lot more for a dollar of honest annual owner earnings than they used to.

The theory is that the thing that has a large bearing on this is - "what are the alternatives?", and this is where interest rates play into it (interest is one of the primary alternatives). If you can buy a bond that yields 5%, will you take a 4% yield on an equity? Maybe, with the expectation of growth ahead that will raise that yield over the years to become higher than the bond alternative. But will you take 3% or 2%? Maybe not because even if there is growth, it may take a long time for it to first catch up and then perhaps surpass, and more time adds more risk (NOTE: This has been shown to be mostly wrong when averaged, time tends to reduce risk, not increase it). So under that theory, in 1980 when rates were quite high, presumably people weren't willing to pay a lot for a dollar of earnings. And over the next ~40 years as interest rates made their way down to near-zero, people were more and more willing to pay more for a dollar of earnings.

There is also a second theory that a whole generation got "used to" such low interest rates and habitually are willing to pay more for a dollar of earnings. But as interest rates rise, perhaps that habit will wane. We shall see.