It's more than that.
If the book grows by one page, that changes the page count. If I remember correctly, that means they'd need to add 3 more pages since it's done in multiples of 4. That means more changes to the book, which starts to add up. What began as the addition of errata has turned into more.
That is not going to happen.
Furthermore, paper publishing is a lot more complicated than it seems at first glance. Aside from formatting changes, which are much more daunting than they seem, there are a lot more factors that go into it. Just working as a bookstore clerk for a couple years and not knowing the full details, I can see that publishing a new printing every month would be disastrous for WotC's sales. If you want to keep bookstores sane, you'd need a new ISBN with every printing. That would complicate things considerably. If you don't get a new ISBN, sellers would be screaming at you to get one. There's some cost-benefit trade-off to consider, too. Once you publish a new printing, your entire last month's stock becomes obsolete. All of the investment you just put into that printing is essentially gone, and you not only have to replace the cost of that, but try to encourage people to actually buy the new book. And I'll go ahead and say that I still have a first printing PHB and don't see myself buying a new one any time soon. Maybe not the entire time 4e is around. Printing a new version every month could actually decrease sales as people wait for the "big" update months to buy a book and skip out on the "small" months. It's a very complicated process, so please cut WotC some slack. They are a business, after all, and one we like to see making wise business decisions.
Granted, digital publishing is another beast entirely. I'm not sure what sort of complications arise there, especially considering its interaction with paper publishing. However, no .pdfs are currently being published (unless they began again and I missed that bit of news).
Edit:
webrunner said:
They should, at least, make a "pastable" version of the errata: each one with cut-lines around it, made to fit exactly in the slot the old one fit, even if that means the font size has to be lowered.
Yeah, this would be nice. They started printing updates in the standard book formats a little while ago. Cut and paste could work for a few things, like power blocks, but I'd love to see them do it for all updates. Maybe even make a separate "pasteable" version.