Because if you don't, you're essentially saying God made a mistake.
The human immune system is a system of & within nature & is therefore a highly intelligent, ever evolving, learning system that is inter-related & inter-functioning with all other systems in the body & in nature.
Why would the Creator who keeps the planets & stars orbiting in perfect unison & harmony snooze on your immune system? Do you think your Creator made you deficient or any less perfect?
Hasha, what an insult to The Divine!
فَبِأَىِّ ءَالَآءِ رَبِّكُمَا تُكَذِّبَانِ
So which of your Lord's favors do you deny?