I think the disconnect is the fact that the game is trying to draw from two genres that while similar have avery differnet outlook on "Endgame."
On one hand you have mmo gameplay and prestige saying that grouping is required and isnt it great to share with your 4-39 comrades in going above and beyond what you can do via teamwork? No one talks about prestige for like... soloing world quests in wow. Its 5 man mythic+ time/level or world forst mythic raid clear. It feels like fractured is leaning more into this side to me.
Then you have your sandbox side. Usually grouping is good here for speed, efficiency, and just the social aspects but it isnt REAUIRED. You buy a Rust server and it literally has one slot for you. You can do 100% of the content eventually. You can build everything you can harvest anything ect. Just takes more time then if you split it with a buddy. I think this is where the frustration of solo players is coming from.
This isnt to say that solo players cant happen in other mmos but thats done with the foreknowledge of what to expect, in that the final pieces of endgame arnt all open to you.