Because it makes more money this way. "Cheap", "Expensive" and "Middle ground" is the optimal marketing strategy.
It feigns transparency but really it is just an opportunity to exploit a common bug in human-brain logic. Specifically, Decoy Pricing.
The i5 and i7 are both quad-core CPUs clocked at no more than 100MHz away from each other. But the i7 is $100 more to make the i5 look like a good deal. When consumers buy the i5 they "think" they got a good deal, when in reality, Intel priced the i7 to make them feel this way.
Read up on Decoy Pricing to learn about this effect: https://en.wikipedia.org/wiki/Pricing_strategies#Decoy_prici...
https://en.wikipedia.org/wiki/Decoy_effect
Also, don't complain about marketers taking advantage of human psychology. That's their freaking job. Instead, you should take advantage of it too.