I mean the work on the season probably isn't done yet. Also, animation like this takes a long time. I think Kirkman and Skybound already said the breaks between seasons would be quicker and voice work is already done on Season 3.
Once again, producing animation like this on this level takes a long time, and the episodes are one hour long. Normally, they'd be half an hour. So keep in mind that eight episodes for this season is more like 16 for your typical half-hour primetime animated series.
Season 2 of Prime Video’s Invincible isn’t even out yet, but Season 3 is confirmed to have already passed a major production milestone. After first arriving back in early 2021, fans have since been waiting over two years for Season 2 of Invincible to finally come about. And while that release...
comicbook.com
Doing the first season they didn't know this was going to be a success. Kirkman kicked Invincible around Hollywood for years. This show is his baby.