Since BC leveling is quite straightforward and having +10ilvl than everyone else doesn't really give you a significant edge. Spending days, weeks even months getting those +10ilvls for a whole 10-20 hours playtime sounds like a waste of time.
Also being geared like a battecruiser doesn't really make a difference while Bobby Greengear just tag mobs faster than you because his class / spec does it better regardless of gear.
Then, most WoW xpac releases happen during evening, if you're so focused on getting ahead of the pack, just drink coffee. It'll work way better than running AoE sims in a game where having Leatrix skip dialogues will do a better job for you than switching gems.
My Burning Crusade time was a long time ago, so I don't remember how much the Nax gear made. Plus the first 8 hours were in a full Nax geared group with a druid, so we were the ones getting the tags. I don't remember doing the same amount of prep as I did in later xpacs, but I didn't play alts at that time, so I doubt there was much more I could have done.
Being the only person in Blade's Edge Mountains for a few hours was a memorable experience though (then I had to go back and help guildies catch up).
There were other rewards for getting to max level fast, outside of avoiding the crowds, that was just my primary motivation. The sooner you are at cap, the sooner you can get started on raid prep. Of course bliz over the years nerfed many of those benefits, so it hardly matters if you are max level in 5 hours or 10.
In DF I will do about the same amount of prep I did for Shadowlands, which is, make sure I have enough gliders and try to remember to use up any old flasks I have in my bank.
As for anyone spending more than a few hours to prep, its mostly is in relation to mythic raiders (and maybe even some mostly heroic raiders), where for the last 4 or so xpacs, you are expected to have multiple characters ready for splits on the first week of heroic. Which means they all need to get to max as fast as possible in order to get the first weeks wizard chores done, since it all adds up in those first three to four weeks.
You can also make a shit ton of gold by leveling as fast as possible and getting the higher level mats and consumables up on the market...or at least you could. They made the AH regional now, for that shit, so who knows how it will play out.
That's not me though, I spent maybe a couple hours reading the prep sheets and rounding up supplies. I never took alts to PuG raids just to gear them up for leveling, but I know people that did (usually they wanted to change mains/primary alts, after seeing what Bliz did to the class in beta).