They do.
The likely scenario is that Apple has been running their custom silicon on Mac prototypes FOR YEARS in their labs. They are probably also running data center hardware with AS; the latter will never make it to market.
However, the low lying fruit is the MacBook/MacBook Air. This is the easiest jump from the A-series mobile device SoCs to a PC processor with a larger TDP.
Apple cannot release an AS Mac that is just as good as a comparable model from 12-24 months ago. It needs to be a substantial improvement.
My guess is that they reached this point in 2017 with the woefully underperforming fanless MacBook 2017. They probably had something in the lab that performed similarly with less power. As Johny Sroudji said, it's all about performance-per-watt. Three years ago Apple likely beat what Intel could bring to the table.
It's not like Tim, Jeff, Phil, Johny, and Craig came up with Apple Silicon over a couple of pitchers of beer last summer at BJ's.
Remember that iPhone wasn't originally a phone. It was a tablet PC by design, perhaps something like the failed Newton. Steve drove it into a smartphone. My guess is that the first seeds of Apple Silicon were planted back in the early-mid 2000s.