Because the standard of 'notability' is bogus when applied to something of the supposed scope of wikipedia. It's impossible to apply it with any reasonable consistency, and it always boils down to a few biased opinions, which is why the topic of notability deletions is so sensitive.
Not every company is notable, and certainly not companies at the two-guys-and-no-revenue stage (compare to your local dry cleaner, who is probably actually profitable).
This list would provide a better understanding of YC. Given that there is no SEO juice with links from Wikipedia and given that being on the list resulted in 2-10 vists per day for RescueTime (when we were on it), I see lots of benefit (in term of being accurate/complete), no harm, and no real significant benefit to the companies lists.
If the information is public I don't see any good reason why it must be a subset.
I would think that either you'd include a list that is as complete as possible, or not include it at all. Doing a partial list because you don't WANT it complete is just warped.