Because comics are created by human beings, and human beings make mistakes–especially across so many titles every month. And honestly, keeping all of the rows absolutely straight and neat isn’t anywhere near as important as telling strong, engaging, compelling stories. Very few people come back the next month just because the continuity was all in line.
So it’s a question of the cost to benefit ratio. In years past, there were people who spent an inordinate amount of time and effort trying to keep everything straight–and the end result was that there were almost as many continuity gaffs, and the actual stories weren’t as good. That’s like shooting yourself in the foot repeatedly.
It’s great that there’s a fan out there who’s taken it upon himself to build his own chronology–anybody who cares about this stuff that strongly has all of the tools that they need to do that. But that’s a passion project.