Because it's a non-issue.
You write as though running faster just means moving your legs faster when it doesn't.
What it would involve is longer, bounding strides and possibly rocket assistance.
Even simply using powerisers will get the average person running at 30kmh, add artificial muscles and computer stabilization to the mix and 40kmh would be nothing special.
Contemporary jetpacks will get you to over 100kmh and burn for over thirty seconds.
Heat buildup was never a big problem, exhaust gas heat can and usually is channeled away for the wearer and jet exhaust can be mixed with cooler air, making it safe for even for an unprotected wearer.
And that's based on cold war research and development.
Where did you get the idea that it would weigh that much?
Who cares about internal volume?
You could just mount propellant tanks externally and drop them when they're exhausted.
Again it looks like you didn't read the book.
The marauder armor flies in long, low altitude bounds, not like a helicopter or something like that.
You are pulling numbers out of your ass.
Rocket belts built in real life didn't even need cooling systems at all and bigger jetpacks (as would be necessary for propelling a few hundred kilos of soldier) would easily be fitted with them.
There doesn't need to be.
If the movement is via pneumatic or hydraulic systems you can pressurize them with an internal combustion engine that could also be used to top up the batteries.
Have it share the same fuel as the rocket pack and you'll have as much operational time as you can carry.
Certainly not, but again, that isn't the point: bouncing a few hundred meters from cover to cover is what it was there for.
Read the book.
Regular infantry are still used for holding ground and grunt work.
The MI were used as a replacement for mechanized and motorized infantry, not infantry altogether.
Now look at the cost and maintenance of an AFV.
I completely agree.
One hundred years ago a tank was just a tractor with some barely bulletproof armor and a low-velocity cannon thrown on.
It took decades of further development before they were able to operate with any degree of reliability, outpace a man on foot, travel any appreciable distance without running out of fuel, carry radios and so on.