I understand WeChat is more comparable to Google services (webview, payments, maps, social platform, chat) in terms of scope, but in America, Google gets challenged in specific areas and loses, like losing social to Facebook after many social media platform attempts.

Why doesn't WeChat get more competitors in just a specific area, like chat, which is a lot simpler, so a company could potentially do it better than WeChat?

WeChat doesn't get that kind of competition in China at all, and it really looks like its because of the government.