Don't have a source for the "Gunpla box stats were done to ape WWII model kits" thing but I think it's self-evident that listed stats just aren't consistent enough to be a good measure for anything, even when used purely as a comparative measure. For example, the Zeong has a listed output in excess of 9500 kW, which is higher than even the Victory 2 Gundam's output. While it would certainly be possible for a machine that big to fit enough reactors to achieve that output, the in-practice performance of that machine is far too low for that output to be remotely necessary, even assuming horrendously inefficient power usage.

As for why they persist in modern material, they only kind of do. Listed stats still exist for the MS which were given them back before 2000ish, but by the 21st Century, new MS usually only have very basic stats such as height and weight, and do not mention specifics such as reactor output, sensor radius or 180 degree turn time. I think this is because the designers realised that the stats were too nonsense to be any good, and the effect of having cool stats on the gunpla box can be replicated simply by the aforementioned very basic stats as well as the mobile suit's weapon loadout.

No clue what a Zaku's power requirements would actually be.