Because Aegir isn’t merely underpowered, he’s off the charts underpowered.
He’s not like an older classic hero that has been slowly getting weaker as new heroes came out, and was at one time serviceable, or who may have had a design flaw exposed as the game develops, he’s been complete trash since day 1.
His ability is not just in need of a re-balancing tweak like +% to damage, or adjustments to stats that would help make most other underpowered heroes viable again, he needs a complete overhaul.
His special is not something standard in the game, it’s unique and very theoretical or experimental in nature but that theory has not panned out in practice.
In fact many of us think he might be a net negative to the team. So it’s not just that he doesn’t add as much firepower as he should, but that he actively harms your team and subtracts firepower.
This is the reason why you see so many threads about him and this thread has over 100 votes where you don’t see anything like that with other underpowered heroes. He is not just bad, he’s exceptionally bad.