I guess it's because, if I go through the effort of creating an excel spreadsheet, what's the point of paying for the CB then? I can just use the spreadsheet and be done with it.
Yes, and?
Save yourself some money and have a nice spreadsheet that you can use any time for the investment of some time and effort and occasionally checking errata...
Or pay for WotCs insta-updating errata and sacrifice offline utility...at least for now.
I don't see it as a contest, even with the old CB- errata updates don't thrill me that much. Much of what I've seen of it should have been caught before the books got printed, and I can take care of that in my head without rewarding a seemingly lax proofreading process.