Let me give you a comprehensive answer on this that goes back to first principles. I've worked on apps like SnapChat, so I am probably pretty close to an authority on why apps like that don't use p2p.

The first problem is that mobile devices are pretty much inherently asynchronous. There are apps that you would use at the same time as another person (like real-time games) but especially on cellular, lag is an issue. This pushes people into designing products that can tolerate lag measured in seconds (because that isn't shockingly bad performance on cellular networks for apps that use your standard off-the-shelf tools like REST/HTTPS/AWS for example). This produces a lot of asynchronous, or semi-asynchronous applications.

Now partly due to those product designs, and partly due to people having lives, they use these apps asynchronously. You pull out SnapChat, fire off a message, and go back to reading Reddit or whatever. Snapchat is off. There's no way to reach you.

Okay, so why don't we run SnapChat in the background? Well there are layers of reasons. The first layer is that it costs energy, and the mobile revolution is in large part possible because software and hardware developers got very aggressive about energy management. If we ran things in the background like you do on your laptop it would need to be as big as your laptop and plugged in regularly like your laptop. There are also practical problems, like announcing every time your network address changes, or even figuring out when your network address changes, which is hard to do passively. I'm glossing over some networking details but there's a deep principle here that within the design of existing TCP/IP/cellular stack you can't have reliability, energy-efficiency, and decentralization. You must pick 2.

Apple, very presciently IMHO, has decided to legislate a lot of rules about background processes that I don't have time to go into here but basically they try to regulate the types of work that apps can do in the background to only the energy-efficient kind. The rules are actually pretty well-designed but they're still rules and they restrict what you can do. Android doesn't have this limitation but unless your product is Android-only you're going to comply with the iOS rules when you design a feature that communicates across platforms.

Okay, so we can't run Snapchat in the background. But what if two users happen to have it open? We can use p2p then right?

Well sure. But the user may be on a network that blocks p2p traffic. That is their network's fault, but they still e-mail you to complain, and leave bad reviews for your product, because as far as they can see "the Internet works" so it's your app's fault.

So what you do is you design a scheme that uses p2p by default and client-server as a fallback. There are actually apps that work like this. Problem here is, instead of getting support tickets about it not working, now you get support tickets about it being slow.

And there are ways to solve this, like permanently giving up on p2p after a certain numbers of failures for example. But the first experience is still pretty bad, which is what counts in mobile. And I remind you, this p2p feature is already a scheme that only works in the 0.3% of cases that users actually have the app open at the same time, and now you want to add code that disables p2p in even more cases than it's disabled already. This process continues until basically zero actual customers will ever use the feature.

And we haven't even gotten to cases like "Why didn't this message get delivered to all my devices?" because there is just zero chance that any customer, anywhere, will have all his devices turned on at the right time to receive incoming p2p connections.

Now non-messaging products like Spotify or Netflix are more plausible, but you still have to ask who wins here. Customer experience is worse, both because of connectivity problems and increasing bandwidth bills and the energy efficiency losses that comes with rebroadcasting content to other users. Developers are worse because they probably have to build both client-server and p2p architecture, because p2p isn't reliable enough on its own. Support is worse because almost any issue is potentially a p2p-related issue, have you tried disabling p2p and seeing if the issue persists?

There's really no reason, certainly no compelling business case, to inflict that much pain on any mobile product I can think of. I mean, there's probably a place where p2p makes sense--we live in a big world--but in general it makes things much worse for everybody.