I have written to Kayak suggesting this in the past. Without such a feature, the hidden charges and nickle-and-diming will just get worse.
The best explanation I heard (from elsewhere) was that if Kayak automatically included any fees, then some users might see a lower price on another site and mistakenly conclude the Kayak didn't actually have the lowest prices. But this seems obviously solvable by making it a non-default option and displaying the price as "$X ($Y fare + $Z fees)".
They could provide a search option to let you plug in what kind of baggage you wanted. So you could say 1 overhead, 2 checked bags, and then it could add in the cost (or show it broken out next to total cost)
The best explanation I heard (from elsewhere) was that if Kayak automatically included any fees, then some users might see a lower price on another site and mistakenly conclude the Kayak didn't actually have the lowest prices. But this seems obviously solvable by making it a non-default option and displaying the price as "$X ($Y fare + $Z fees)".