That's because all the "engineers" who are not working at Apple are just soooo much better than all the real engineers who _are_ working at Apple.
I'm sure _you_ could fit an i5 with discrete graphics into the 13" case with 10 hour battery life and sell it for $1199 at a profit that keeps shareholders happy. You can, but only as long as you post on MacRumors and don't have to do the real work.
Of course every real computer will have compromises in its design (you don't want to end up with something like the car designed by Homer Simpson). The specific compromises that Apple made in the MBP 13" are actually not the ones that I would have hoped for for my personal computer use, but they are exactly the ones that I would have used to sell the maximum number of computers. The 15" MBP is there for people who prefer different compromises.
The 13" MBP has more CPU performance than 90% of computer users will ever need (and there is the 15", the iMac i5 and the MacPro for the other 10%), it has enough graphics performance for 90% of computer users, it is light enough for 90% of computer users and Apple builds the MacBook Air for the others, and the 10 hour battery life is the best around. And an MBP 13" with comparable specs sold for $300 more just two weeks ago. No complaints at all.
Read that post again. I'm sure English is his first language. Nobody learning English as a second or third language would start every sentence with a lowercase letter. "kidding" is not a word that is in your average English book for beginners. It is the typical careless and lazy approach to writing that you only get from people using their first language. And it is _very_ annoying to anybody who is learning the language, who finds it a lot, lot harder to understand a post when it isn't written in correct English, and who isn't sure about what is correct and may even copy errors if they read them often enough.