Scuderia Ferrari are the only team to have competed in every season of the Formula One World Championship since its inception in 1950. However, during the 2019 Chinese Grand Prix, the 1000th F1 race, a TV graphic appeared indicating that Ferrari had only taken part in around 970-something. I was surprised to learn that they'd missed that many.
As of the 2019 Mexican GP (the 1015th F1 race), Wikipedia records the Scuderia as having entered 990 races, and started 987. That leaves 25 races that they didn't enter, and another three that they entered but never started.
What were these 28 races, and why did Scuderia Ferrari not compete in them?