Because bacteria are easily detected, formulators and end-users are aware of the need and benefits of using a bactericide on a regular basis. Both types of bacteria thrive in the metalworking fluid itself, where they disperse evenly throughout the system. But they take in nutrients voraciously. As a result they reproduce very rapidly. In the real world, growth rates would be somewhat slower though still tremendously rapid by any standard.
Fungi are hard to detect.
It is sometimes assumed that because fungi have not been found in a fluid sample, none are present - and no fungicide is necessary. This is not the case. Fungi are almost always present in a fluid because they are commonly present in the air to which the fluid is exposed and they may also be introduced via water, tramp oil or a variety of other ways. However, they are usually fewer in number and less evenly distributed in the fluid than bacteria. As a result, contamination is difficult to accurately measure by conventional dipstick or plate count methods.