I have a question that I did not find an answer to online and maybe anyone here could help. Why are there still prokaryotes today? Why didn't they all evolove?
They did evolve, and they keep evolving every single day, like all other organisms.
More fundamentally, what is evolution (in a classic Darwinian definition)? Evolution is 1) the random apparition of new traits and 2) the selection of some of these traits because they provide some kind of advantage - antibiotic resistance is a perfect example. Prokaryotes mutate much, much more than larger organisms like us, so they constantly try new biological strategies, and because their generation time is short, selection occurs much faster. So prokaryotes actually evolve faster than most eukaryotes!
I think you are confusing "evolving" with "becoming more complex", and then "becoming more complex" with "being larger and multicellular". Both of these are incorrect.
Prokaryotes live everywhere on the planet, even in places where animals and plants struggle to survive. They can live in water, in air, in the cold or in extreme heat, in acid, in pure brine... When you think about it, micro-organisms really rock! They can adapt to conditions much harsher than what we can tolerate. This ability to adapt is pretty much the definition of successful evolution.
I'll conclude with a bad analogy: what do you think is the most "advanced": an iPhone, or a big gaming desktop computer? They're both recent, they can do lots of things (some identical, some different) but one if not per se better than the other, they simply have different ways of being awesome, in different contexts.