The spline teeth that need the grease, are INSIDE the female coupling, and turn well clear - perhaps 10mm? - of the inner walls of the swingarm tube.
A grease nipple in the swingarm
tube would simply pump huge amounts of grease into the tube, right up as far as the engine, and it would be most unlikely that ANY of it would get into the splines where it's needed.
You would end up with a swingarm tube full of grease, which is no use to man or dog, and quite probably STILL run the risk of dry splines.
Thought about this years ago. Can't see any possible benefit.
Which is very sad!!!