Google building another App like WhatsApp is a thought that the Google Company would have most likely thought of themselves. As the reasons might be best known to them, some of the reasons would be highlighted in this article. First of all, Google is the most used search engine; then, have you thought of the reason why Google Company did not build another similar search engine?
Similarly, they are successful at having the WhatsApp mobile application, which also happened to be the most used social medium in the whole wide world. Now, a very major reason is that, if they should build any other similar social medium like WhatsApp, then people would definitely lose focus on WhatsApp.
The reputation that WhatsApp Mobile App has given to Google Inc would be shared between two platforms. And then another organization could come up and have just a similar Mobile App, and of which it either takes the lead or be a close counterpart with WhatsApp app just because Google themselves share its (WhatsApp) reputation with another.