Why did Mike and Carol not have kids together?

6 kids on one income plus a maid was probably more than enough!

They were too old.

Why? Just to have pointless scenes with a baby that add nothing to the show and take time away from the story-lines?

So many reasons

a) They never had sex (because Robert Reed way gay)

b) Mom said "no playing with Mike's balls in the house!"

c) Too much of an age gap between Bobby/Cindy and the new baby

d) No room in that house for a nursery

With four kids entering puberty I don't think Mike and Carol had any inkling to produce an infant child and bringing into the mix. They already have little children with Cindy and Bobby and as the series progressed when they added another child in cousin Oliver it added nothing to the family dynamic.

Well if they had just two more and stopped, then the show would have been 8 is Enough

All nine boxes were taken in the opening credits.

step by step did this.