How come men get aspermia ?

Two ways ...: Azoospermia (no sperm or no living sperm) could be due to obstruction (blockage) or a sperm production issue. It should be confirmed by more than one semen analysis: a fertility clinic will 'spin down' a sample with no sperm so that we find low numbers of sperm. Many men still have some sperm production, so sperm can be surgically retrieved for ivf with icsi, or use donor sperm. See a fertility md.

Also retrograde: Agree with the prior answer. Another reason, although less common, is retrograde ejaculation in which semen goes into the bladder during ejaculation. It's not at all common in the general population, but seen with some frequency in diabetic men.