Your knowledge is lacking.

Yeah, that's true. It's an oversight. I'm watching a video by some guy named LeBron, and he's upgrading skeletons.
For speed? But what about necromancy? Are you supposed to run around upgrading them later? Although, if you build a good chain of heroes... Experienced players have 5 heroes by day 2. I'm not arguing, I'm actually trying to understand the game's mechanics. Especially for Necropolis, since it's my favorite faction.
What about Vidomina and Gatran?