Why does the speed of earth's rotation not affect us?

Because the rotation speed isn't fast enough.

The centrifugal force is the fictitious force seen in a rotating reference frame. It has a magnitude of ##F_C=momega^2r## The angular speed has a value of ##omega=(2pi)/(24h)=pi/(12*60*60sec)=7.27*10^(-5)(rad)/s## Earth's radius is ##r=6,37*10^6m## So ##F_C=m(7.27*10^(-5)(rad)/s)^2 6,37*10^6m=m*0,034m/s^2## Compared to gravitational force ##F_C/(mg)=(0,034)/(9,81)=0,0034~~10^-3## The centripetal force is one thousand weaker than the gravitational force, so we don't see its effect in the most situation. For a situation in which this effect was important look for the first Falkland's Battle.