The big reason is that we're all impostors. You're not imagining that you're an impostor, it's likely that you are one.
Everyone who is doing important work is working on something that might not work. And it's extremely likely that they're also not the very best qualified person on the planet to be doing that work.
How could it be any other way? The odds that a pure meritocracy chose you and you alone to inhabit your spot on the ladder is worthy of Dunning-Kruger status. You've been getting lucky breaks for a long time. We all have.
Yes, you're an impostor. So am I and so is everyone else. Superman still lives on Krypton and the rest of us are just doing our best.
Isn't doing your best all you can do? Dropping the narrative of the impostor isn't arrogant, it's merely a useful way to get your work done without giving into Resistance.
Time spent fretting about our status as impostors is time away from dancing with our fear, from leading and from doing work that matters.