The more drawn out answer is the older legal mistakes get, the more expensive, frustrating and dangerous they become to fix. A simple mistake in your articles of incorporation can cause serious pain down the road, even potentially derailing your company all together.
Therefore for most "serious" entrepreneurs, it's much cheaper in the long term to just get incorporation done right from the get go. So is it really worth it to take that risk to save $1000?
All that being said - I'm not a lawyer - but I've been told by my lawyer that if you're a sole entrepreneur without any partners or investors, the legal side of incorporation is pretty straightforward, and you could probably get away with using a discount automated service provider. Things get complex once you have partners, shareholder agreements, employee options, investors, and so fourth.
This means that their boilerplate is geared for having seed investors with an easy ability to add on larger investors later on.
In short, their boilerplate documents fit the way that they do business, and as such, is probably not a horrible way to go if you're going to do business with YC. If you're not going to do business with YC, then there are a million other things you have to account for. Even if you ARE going to do business with YC, you still might want to change the documents in play to fit your environment best. As I understand it, there's enough experience there to be able to guide you as to whether or not their boilerplate fits your exact needs.
Therefore, they tend to have radically different needs than the type of small business Legal Zoom targets with its services