[crazygringo]

Pure speculation, but at some point your dataset is large enough.

The original reCAPTCHA corrected errors in scanned books published decades/centuries ago. At some point, they're all fixed.

Similarly, more recent images have all been of traffic images. And they probably have way more than enough now -- at least of the type that can be done by reCAPTCHA.

So unless Google comes up with a new mass-categorization problem easy enough for literally everyone to do and simple and small enough to fit in a reCAPTCHA... then they charge.