Why is reupholstering an armchair more expensive than buying a new one? And can I do it myself?

It made me sad to see that they charge way more to reupholster a chair than getting a brand new one. I get that it's a lot of labor, but that means people are going to just throw their old ones away and buy new instead. So I'm wondering if its something I could do myself? I don't care about the labor time, as long as the result is acceptable. The materials aren't too expensive. Has anyone here reupholstered furniture and have any tips? I feel like it should be totally doable.

Edit: Thank you so much for all responses, I've read most of them! I can't reply to them all but the consensus seems to be that yes it's doable but time consuming and a bit tricky. I obviously am not trying to say that upholsteries are overcharging or that it's an easy job, just that it's sad that most people will buy new since it's cheaper. I will never do as good of a job as an expert obviously.

I've learned so much about it though and it looks harder than I initially thought, but doable. I found this detailed guide on it if anyone else is interested. He's not selling anything in the video so I think it's okay to post here? It's only focused on the upholstery. Remove the link if it's not allowed but I found it super helpful!