A pulsar like PSR J1478 is a very newly born star, sadly there is not much information on its age.. BUT, I can tell you about why it spins so fast. So for example, take a large star, and imagine that its mass CANNOT change at all. So with your star, you will shrink it to around 20 KM in radius. Assuming the star was already spinning before becoming a pulsar it'll spin at the same speed tho having a seemingly smaller size of around 20 KM because the mass is still the same. Because being so closed to shrinking to the point of becoming a black hole, if it pulls in enough matter (forgot to mention the extremely high pull on pulsars) it'll just become a black hole. In my opinion, its either because their extremely small size or because they all became black holes millions of years before the discovery of the telescope. Well, they're already extremely hard enough to see with a telescope and impossible to see with the naked eye so thats also a possibility.
