It's been less than a century since the widespread use of antibotics started, and already we're seeing bacteria that have evolved immunities to the antibotics we use.
On the other hand, we've been using immune systems to to fight bacteria for millions of years, and bacteria evolve much faster than humans do. Why have bacteria not evolved immunities that let them completely overwhelm our immune systems and kill us all?
Answer
First of, not all infections are mediated by bacteria and not all bacteria are infectious. Also, not all parasitic bacteria lead to a strong infection or important health problem that cause any non-negligible selection pressure. Following the implicit logic of your post I will talk about infectious bacteria which cause "noticeable harm".
The mistake in your thinking is in the sentence
bacteria evolve much faster than humans do
While it is true that the mutation rate of base pair is higher and that the generation time is lower, the statement bacteria evolve much faster than humans do is made way too general and lead to your mistake here.
Infectious bacteria are selected to deal with the host immune system and the host is also selected to deal with the the parasite. Both virulence and penetrance evolve in this evolutionary arms race.
The evolution of host-parasite interaction is a very large field of study and it would take several books (and several books have been written on the subject; see this Amazon search) to summarize it. For some theoretical insights into the evolution of host and parasites, you might be interested in the book by Martin Nowak Evolutionary Dynamics: Exploring the equations of life
You will also note that a parasite does not have any "direct interest" in killing (or even harming) its host (thanks @jamesqf for his comment). You might want to have a look at the post Why do parasites sometimes kill their hosts?
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