There are a few reasons. Some already stated.
1) if your building a motor it is most likely going to be very high HP and many times will use a power adder. Even if its not using a power adder and is staying NA most will spin the crap out of it and have high compression. RPM is just as big a killer of motors as high cylinder pressure. So I think we have a tendancy to push it right to the maximum and just beyond.
2) build quility and tolerences. Regardless of who is building your motor...many times it is not equal to the process and quality control of a major manufacturer. Many mistakes leading to an engines early death happen...even to the best engine builders.

I think if you took a built motor like mine and kept HP around 430 rwhp and 425 rwtq and only spun it to 6300 like I do and didnt spray the crap out of it( like I plan too) it would probably last 100k. But seeing as how I hope to occaisionally see upwards of 700 rwhp.......