Rover.com is doing pretty well [1] with a similar service. In my area at least, these dog sitters (and, I assume, these dog walkers) are young, and take it on temporarily as an alternate source of income. They may not stick around for long, and you'll soon be looking for another. Also, their schedules may change frequently - your preferred walker/sitter may not be available on the date of your vacation or other cause for needing the service. And for the price, both the sitter and the walker may find keeping track of billing and payment through the app to be simpler.

[1]: https://www.bizjournals.com/seattle/blog/techflash/2016/10/r...