This game runs like poop on anything but newer hardware so I highly doubt the crappy UI was developers being considerate of small screens and struggling CPUs and instead it's probably because Bethesda games always have a shit inventory UI. This game's inventory UI is a small step up from Fallout 4 but still no where near as good as the DEF_HUD or FallUI mods for Fallout 4.

This is the real answer

Part of that is the ability to focus on improving on all of the work that already exists and the other part is being able to unilaterally make decisions while being emotionally invested.

Developers are often spending the majority of the time making sure whatever gameplay exists passes testing without breaking the overall compromise vision that is limited by time and money for deadlines. Companies don't allow enough time for polish and frequently have decisions made based on the added cost of labor and testing tha mods don't have since mods having bugs is an acceptable situation for people while a company putting out bugs is generally met with hostility. Different levels of standards and costs have a huge impact on why many mods vastly improve the game in ways that don't fit into the game development process.

Plus a lot of mods are focused on a specific part and don't appeal to the playerbase as a whole.