Can some one explain BCD?

[cce]

144 still common on the track where the big guys use some absolute monster chainrings

On the track, why are they not using a fully integrated chainset ?
Where the ring is the crank itself?

I don't follow track or road much (or mtb these days much unless someone asks about it or I need a new part.)

They are at the highest levels, but for mortals it's useful to be able to change ratios