The question in the title makes a presumption that there is more bacteria in the mouths of cats than in the mouths of people. That might not be true.
Mouths of humans, cats, dogs and any other animal are excellent places for bacteria to live and multiply because they are warm, dark, wet and there’s food in there. Many bacteria eat starches and sugars which is found on nearly all foods in the mouth of animals including the human animal.
The main bacterium in cats’ mouth is Pasteurella multocida (90% of domestic cats). But, I am told that there are over 600 types of bacteria which can live in the mouths of humans, dogs and I presume cats.
The fact of the matter is that domestic cats are not special cases. They don’t suffer from a rampant colony of bacteria far in excess of which is found in dogs or humans except for the obvious fact that people normally brush their teeth and sometimes use anti-bacterial mouth washes. For this reason only there is less bacteria in human mouths than in cats’ and dogs’ mouths unless you brush their teeth too, which is unlikely.
What might make cats stand out from the pack in terms of getting an infection from a cat bite is that they are able to ‘inject’ the bacteria under the skin with their strong bite and relatively sharp canine teeth. Applying common sense it is much harder for a person to break the skin of another person and inject bacteria deep into the skin as we use incisor teeth and they are fairly blunt (our canines are very short). Also several incisor teeth are used when a person bites another person. This spreads the load at the tip of the teeth.
Applying simple physics, the amount of force applied at the tip of human incisor teeth when biting is much less than for a couple of pointed canine teeth. Therefore human incissors are less likely to penetrate.
Also, of course, domestic cats are top predators who instinctively use their teeth all the time. They are great potential biters. This concludes the discussion on why do cats have so much bacteria in their mouths.
I have to make one last point, however. Cat owners know that they can nearly always avoid a cat bite. It’s about how we interact with cats. You have to learn the nuanced body language, emotions and habits of domestic cats to predict when they might be in the mood or frame of mind to bite. You then respect the cat and avoid those moments.