A pretty simple answer is that google scholar weights citations highest of all search criteria in its rankings. Citing your own work (where appropriate) increases its rank order under google scholar's search results. You increase a few other metrics with self-citation including your h-index. This seems like a pretty straightforward incentive for scholars to self-cite themselves.
From a practical point of view, though, why would you not cite yourself? If you are producing quality research in your sub-discipline, not citing yourself would be inappropriate. This is especially the case if you have a strong and directed body of research.
Here are relevant citations:
Beel, J., & Gipp, B. (2009, April). Google Scholar's ranking algorithm: the impact of citation counts (an empirical study). In Research Challenges in Information Science, 2009. RCIS 2009. Third International Conference on (pp. 439-446). IEEE.
López-Cózar, E. D., Robinson-Garcia, N., & Torres-Salinas, D. (2012). Manipulating Google Scholar citations and Google Scholar metrics: Simple, easy and tempting. arXiv preprint arXiv:1212.0638.