'Why are there still monkeys? and 'Why are there still bacteria?' address different issues. In answer to the first question, there is no claim that the common ancestor of humans and monkeys is extant, unchanged from the time of divergence. There are species, however, which have remained largely unchanged for billions of years. And why is that? The simple answer is success of the species in many different niches. Bacteria, however, are not genetically isolated and extensive lateral transfer of DNA, speaks against genetic insulation required for an unchanging species.
So, the answer to the second question, in fact, is more complex and the question not ridiculous.
I agree that the answer is more complex. However, I'm looking at the question as another way of saying, "If evolution is true, why is there anything other than humans?" -- assuming humans are as good as it gets here, which I doubt. It's a strawman.
You answer the question of why bacteria can adapt. The question which the above does not answer is why if these, the first organisms, were able to adapt, were able to fit themselves into just about every ecological niche there was any reason for them to 'evolve' into something else by transforming themselves into different more complex creatures. Also let's note that because these simplest life forms were so adaptable (and indeed are more adaptable than any other creature after it) don't we have a case here of creatures becoming less fit? Don't we have here a case of deevolution instead of evolution?