Widescreen Gaming Forum

So I've been a vacant member here for a while. But I recently found myself with time on my hand to pursue gaming. And I was suddenly revisited by the nightmares of 21:9 gaming. Mods, Hacks, Edits, External apps, yada yada. And I found myself asking how something that "seems" like an easy fix is somehow not.
I obviously have no delusions as to how difficult it must be to modify these games to do simple things, but from an ignorant's viewpoint, could someone in lamens terms explain to me where modder's usually start when modifying game logic. for example, black letter boxes in games that cover the scene but can disapear under specific circumstances like skipping the cutscene.
Example: My favorite series Tomb Raider is plagued by issues.
TR Legend and Anniversary with the help of FWS still display black letter boxes during cut-scenes that can only be dismissed when in the display, causing cut-scenes to look even wider and shorter. Underworld crops cut-scenes. 2013 crops in full-screen but opens in windowed(not windowed border-less(yes in external border-less)). Rise Letterboxes to 16:9 which disapears when you click skip. and IIRC Shadow does too.
Why is it hard to remove those trivial things? Is it because its deeply embedded in the engine, or because the modding methods used here cannot modify said elements?
I'd like to understand how these things are done a bit more than I currently do(I basically understand nothing)