First off, I'd just like to say it's nice that there is some kind of PVE endgame with a grind at all. It's really repetitive, poorly balanced, and not respectful of the player's time, but if it didn't exist then I'd probably just have backed everything up and uninstalled after finishing chapter 3's story.
All the issues with the dungeons specifically aside, the only real reason to get better graded gear outside of the 5% of the playerbase that does any kind of non-vehicular pvp is just grinding dungeons at higher difficulty to get better graded gear. It's like running grandmaster strikes in Destiny 2, except it's better done solo, not nearly as flashy, and there are no other worthwhile activities to actually use your loot in so you just run more grandmaster strikes.
We need other PVE activities that are meaningfully rewarding and warrant having better gear, especially for groups. Something akin to world bosses, public events, or raids that are big challenges that typically require at least a few people. A weekly landsraad mission to assault an enemy stronghold that gives a ton of contribution, scrip, and specialization xp, as well as schematics, melange, and other goodies from a loot room at the end. Stuff that's meant for a full party and doesn't scale by player count but is technically still doable solo if your gear and skill are up to scratch. Give me an excuse to play with my guildmates and flex my gear, if you're going to embrace the PVE RPG elements of the game you have to commit.