It's not exactly like that. Why FB or Inst is so popular? Because their easy to use. See how many forum applications in the google play store, eg. XDA, AndroidForum, Nielsen, OnePlus, Xiaomi, and more, more...
Why ForumApp is better than website on the smartphone?
Because it can start automatically when you turn on your smartphone and you're already logged in - the website does not.
Because the application can have access to the functions of the smartphone, e.g. notifications and you immediately know what's new in your topics - the website does not.
Because you can use sharing your phone resources like photos, files, links etc. directly into your forum application with one click Share It. - the website does not have such possibilities.
And probably something more. All this is the convenience of the user of the smartphone.
Youth sets development directions and is inevitable. Fortunately, my phpbb-based forum is mostly used by people over 40 years of age and older.
I am looking forward to the development of Progressive Web Apps, maybe it will be a significant improvement for websites.
Personally, I send many posts straight from the smartphone but usually they do not contain photos. I don't know why. Maybe i'm lazy?
I use the forum application of one of the phone manufacturers myself. In the application I have the same forum resources as through the website but it is easier to use the application than the websites on the smartphone.
But do not worry about my thoughts, it's just my subjective opinion. Maybe I am wrong.