This isn't surprising, actually. If you want to prove data is coming from a human, then you need to involve a proof of work system, like the guided tour puzzle or a Hashcash-like implementation, or both.
Encountered one yesterday that required me to assemble a jigsaw puzzle. Approached the threshold of not being worth the effort.
Google's NoCaptcha required me once to select the cakes in a grid of 9 images. Only once, never again. I wonder if they were testing it or they have a rate between using text and images.