That’s because Apple hasn’t charged for OSX/MacOS updates since 2013 with OSX 10.9
Edit: I also don’t think they were ever $99. I upgraded from 10.5 to 10.6 back in 2009 and I think it was only like $30, and they had to ship me a disc with the update so it’s not like they didn’t have any overhead they had to pay for
Ah! Shoulda done more research, my bad. My first MacBook came with Leopard on it and I eventually upgraded it to Snow Leopard. I think that computer eventually died before I could upgrade it any further than that