Because the ingedients, delivery mechanism and introduction site all mean the vaccine has no interference with any part of the reproductive system.
Ivermectin on the otherhand is designed to infiltrate the nervous system of nematodes and disable them. It is remotely possible it can also breach into the human nervous system as well given too high dosage. Being oral delivery means it will spread throughout the body including reproductive system and brain.
BTW millions of pregnent women have now recieved thr vaccine with no side effects on pregnancy. I know of several un vaccinated pregnant women that have died from covid, most in their 30s.