Because there was no need for them to. That is, they evolved to survive to many conditions that exists so perfectly that there is not "natural selection" that is acting on them. Since evolution is all about survival, they don't need to change. Also, evolution doesn't "aim" to make successive species intelligent, so if tomorrow there is another comet strike, "most intelligent" species will be wiped out but cockroaches will survive.. as they did last time...
What are you trying to say, they evolved first and now their evolution stopped because they became perfect??....if you think so then you're contradicting the very theory you're supporting, you're taking science's side but not being scientific/logical in your approach....because Evolution, according to the evolutionists, is a
never-ending process.
In the 'comet strike' example of yours, why and how do you think the cockroaches survived the previous strike while the 'more intelligent' life forms perished despite facing similar conditions??.....it just doesn't add up....
...Also, evolution doesn't "aim" to make successive species intelligent...
....evolution doesn't "aim" to do anything, it is not a process designed by someone.......so, the brain, like any other parts of the body should evolve too....and species should become more and more intelligent/adaptive....infact, there is evidence about how the shape and size of the skulls of the primates changed over time, with that, their intelligence......indicating adaptation of the brain(probably depending on its use, just like any other organs).....but the question is, why did that happen to the primates only, that too, only to a sub-group of primates.....astonishingly, other sub-groups remained more or less same......as I said, things just doesn't add up.......unless of-course, you consider the reasons below.
Time period seems to have no effect on a species intelligence, this applies universally, again humans are the exception and not the norm.
How and why did the humans become an exception.....unexplainable, unless you bring in 'God' who created them, or 'Aliens' who mixed their genes with primates of the earth, you can even consider 'Gods' as 'Aliens'.....but the thing is, Humans shouldn't have become exception....