Apps need to be explicitly programmed with all
of the functionality it provides
Forums are (generally) hosted by underlying forum software.
The app programmer either programs everything into the app, or chooses what functionality to program into the app. The underlying forum software gets updated by their software developers which then means the app falls out of line with the overall capability
When smartphone operating systems are updated they change functions which are used by apps. An upgrade may not affect an app, but after a couple of upgrades there is likely to be a function that is no longer compatible and apps either fail totally or act unexpectedly. An app developer then has to recompile, test and resubmit - which can also render users of older devices without the update unable to use the app
Apps also need to meet the criteria of the relevant app hosting services repository - App Store for apple devices, and there can be additional costs
to submit your app
These have criteria to be met, though for Android a user can opt to choose different repositories. Apples is more stringent and you cannot use others unless you jailbreak your device
The strapline for iPhones and the App Store was “there’s an app for that”, however from the birth of the iPhone there was an app for most of it - a web browser. You can also create an icon for a
web page
Smart phones are capable of managing content from the web, you may get the ‘mobile version’ but can also set a mobile browser to display in ‘desktop mode’
It can be personal preference, some prefer apps, some prefer browsers in mobile mode, some prefer browsers in desktop mode