Well simply put, he saw how successful WeChat was in China and believes that this success can be replicated in the States. Imagine. Having one app for banking, job applications, messaging your friends and following your favourite content creators? And then for those pesky other accounts you could have a password manager on X? Why even have another app? And although it isn't in the cards right now ideally he'll want integration with the government, as that's how WeChat can get all it's records for people (and I further guess he's banking on a Trump government to help him with that).

(yes I realize how horrible this sounds haha, just trying to illustrate how Musk sees this)

There was this app about 12 years ago which was like 'foursqaure for payments!' It would advertise 'blahblah just paid $55 at some lame restaurant'. It seemed like an awful idea and it died, but... currently, Venmo does that.