Can you explain in detail why some mobiles app don't use social logins?

Why would you use them? Social logins tie you to a 3rd party, which adds risk to any product and takes data ownership away from you, so I'd avoid them for an app unless the customer base demanded it.

Not everyone who uses your app uses social media or whatever service you would link for identity. Many of the most vocal and influential people especially, tend to be somewhat eccentric or idiosyncratic, so unless you're sure of who exactly your customers are, you want to spread the widest net possible. Data control and customer privacy are probably issues as well, but if login is the first impression, you probably want it to be as inoffensive as possible.

Speaking from my limited experience developing my own app, 3rd party log-ins are a mess and not all use the same protocol. Most use OAuth but some are proprietary(ish). And to have user signing up in-app, you have so much more control, in terms of informations you need prior to giving the user access. In my app, If a user sign-up with 3rd part service, and have to block the user from getting into the app before I have more info from them(necessary data). I think it's pretty janky.

Privacy...you are giving away info about your app and its users...you have to tell your users their info is going to a third party...you have to agree with a third party they will take and sell info about your users to other parties

[I am not a founder]

Social media login is a dependency.

When social media login breaks, users contact the app maker for support not the social media company.

An app maker doesn't have control over the timing or reasons that social media login might break.

Those reasons can range from a user having their account banned to the platform deprecating an API.

Basically, another company has control of your app's availability and access. Your users may not be able to recover their accounts.

And you take the loss of goodwill.

I worked on a very popular and widely used product used by many demographics of people. We had implemented numerous login options.

About half our users logged in with Google, about half our users logged in with email, about 1% used Facebook login, and the rest were barely touched.

I now implement Google and email login on future projects.

Why should I let you access a trusted account (ie one of my OpenID/social media sources) in your app?

I don't know you, or may not want to - signing up with an email address has worked just fine for 30 years (or, better yet, not signing up)