Why apps on iOS save the files inside the app and not in a folder i choose

This is a user forum, you are not addressing apple here just fellow users

apple don't make the wps office app Multi Platform Office Solution - WPS Office Help Center

best they can do is to ban the wps app from their app store if it does not meet with their quality requirements

you can try contacting apple Support ->  Get Support (apple.com) / Contact - Official Apple

Support

and or give them feedback -> Product Feedback - Apple

iPads/iOS/iPadOS is not a full laptop/desktop OS experience, currently.

It is a tablet OS that is a closed, sandboxed OS, to make the entire device and OS secure.

There is no file directory in iOS/iPadOS and no real access to the iOS/iPadOS file system or system files.

Many apps in iOS/iPadOS will only allow saving data inside of the app/s/

Some apps will be coded to be able to allow saved data to the iOS/iPadOS Files app, instead, but this is the extent to where files are saved.

There is no hierarchical file folder system/directories in iOS/iPadOS, currently.

Sorry.

If you need the full functionality of a laptop/desktop computer, than any iPad is not for you.

Sell your iPad and purchase a Macbook with a full macOS or Windows PC laptop or a Windows based tablet computer that offers s full laptop/desktop OS in a touchscreen tablet device.

Best of Luck to You!