I have searched for storj app on app store and there is no storj app. Why?
Storj is designed to be used by developers for their apps, both existing and new. There are apps that use Storj as well as apps that support S3 connections that work fine with Storj.
Some examples of software working with Storj
This has been already discussed before on other threads, there is at least one app on Google Play which is made by our contractor @utropicmedia-karl Android App provided by and endorsed by STORJ - #3 by utropicmedia-karl - I am not sure if anyone is working on a mobile app for iOS yet. As mentioned by @Knowledge Storj itself is the object storage platform any developer can build apps on top of. Storj Labs does not build these apps though, that is not our focus.
You may also take a look on Photos+, it has app for Android and iOS and macOS: Search results for 'photos+' - Storj Community Forum (official)
For the same reason there is no Amazon S3 app.
Storj is a storage platform. Developers may use it to build apps on it if they choose
To be extra pedantic, technically, there is: AWS Console on the App Store — it supports multiple logins and allows to manage various aspects of Amazon services, including S3-specific, such as Storage Lens.
Whether storj needs similar app to manage the account — debatable, but IMO — not right now. The web app works just fine on mobile and development efforts probably are best spent elsewhere.
In the context of “app to use the storage” — answers have been provided above.
Well, yes. But that more of a management console rather than a use of the service S3 provides.
You know what I meant