From a completely non scientific standpoint, the gears give you a better sense of climbing through speed as opposed to just one sound that increases in pitch as the vehicle gets faster (IE, the Jet engine, or an electric car if one were to be made). In a game you don't have G forces slamming you back into your seat from the acceleration or anything like that, so there are other stimuli needed to make it seem like the vehicle is accelerating at a pretty good rate.
From a more scientific standpoint, IC engine or electric, more than likely these cars will need some form of gearing system, and I will try to explain my thoughts as to why it would make more sense to have them rather than not have them (remember, this is just an opinion and observations and nothing behind any of this really is meant as a counter to your argument... just trying to explain what I see in front of me)
Now... lets start with the first hurdle... speed.
These vehicles can hit tremendous speeds in game. We are talking about 500+ MPH here. If we guess that the tires on these vehicles are 8 ft tall, then that leaves us with a rather low sounding 666.9 RPM's at 500 MPH (I have no idea how big these things are, but I thought 8 feet sounded on the high side, so I went with that as smaller tires would only make the problem I am about to bring up worse). Now, in a different thread, I calculated how much power it would take to sling one of these cars from 0 to 400kph (why I did kph instead of mph I have no idea, but that's about 250mph) in about 3 seconds. Using the weight of an M1 Abrams tank of about 65 tons (which sounded about right to me) I came up with a needed HP of about 163,000 HP. That number is insanely high, but not impossible as I kinda proved in that thread with some math. Anyways, I did those calculations for an IC engine being assisted by Jet engines (also IC but you know what I mean). Naturally then the setup I was going for would have needed a transmission.
If we assume that we scrapped the Engines and went for electric motors, then we would have to have some crazy motors or much better jets than I was assuming. Let's assume that the jets can somehow generate 120,000 HP of the required 160,000 leaving 40,000 HP to still be made up somehow.
I found some 10,000 HP Electric motors on the interwebs that can spin at full load at 715 RPM's. It runs on 420 Amps and 13,200 Volts... that's a lot, but unimportant for what I am doing here. One of these for each tire would get the job done... and actually... kinda wholeheartedly kills my argument. I didn't think that these ultra high HP motors could spin that fast. I thought they would only be able to achieve about 400 RPM's, so in theory, it could be done. Now... the motor I found is about the size of a camper, but even still... I think most of that assembly is for cooling more than anything else so at these speeds, there should be plenty of cooling capacity.
Well, at least you get to watch me go through the process there of discovering that... I am not going to remove the rest of the post because I am lazy, but yes, you could theoretically do it. The only downside would be the cost of that INSANE battery that would be needed to power such a machine, so perhaps some of them run IC's for cost while the rich guys run Electric