You can't make the weapons incomparable or you defeat the purpose. Someone suggested the aliens have defeated bullet-technology. Well then humans would be non-competitive.
Shields protect against high energy weapons, so a kinetic weapon has an advantage there, the mass is much higher than the energy involved. If anything the shield just turns the mass into a molten mass like sabot round.
Aliens still use shields even though tactically they shouldn't.
As for beam-weapons versus kinetic weapons.
Humans don't have a lot of defense against the beam weapons so aliens have no reason to adopt them and already have the beam weapons-industry built so aren't going to retool everything to switch to kinetics.
Humans have the kinetics and against shielded opponents are not a disadvantage so the incentive to develop beam-weapons is much less.
Alien strategies and tactics developed millenia over vast deep space conflicts with little logistical capacity. The colonial wars in far flung worlds require minimal industrial effort or minimal supply lines so beam-weapons are favored.
Humans are on defense so they have access to all the resources and control the access, so they can mine the kinetic materials required.
Other things I saw as answers are also not really that good (not to poo-poo anyone).
Ammo vs. recharging? There's no difference. You can only carry so much ammo. It is MORE LIKELY that an alien can fire many more shots than a human can simply because power density is much greater than mass density.
Microwave power-beaming could remotely recharge your beam weapons in the field.
Tactically, humans could be forced to take out the "microwave battery tank" before really engaging an alien infantry force...so that their ammo can at least be limited.
A beam weapon would have a much higher rate of fire. Suppressive fire would be much more practical than a machine gun.
Machine guns are designed to pepper an area, a beam weapon can be designed to do the same only it'd do it more precisely with a computer generated algorithm that would ensure the most volume of area at a precise distance is filled by a pattern of beams flooding the area per second.
Ultimately, the only advantage a kinetic weapon has is it "does the job".
Because beam weapons take so much more to integrate, use, and maintain (such as the microwave battery tanks to recharge), a soldier might capture beam weapons but only be able to use them a limited time.
Developing beam weapon technology can be a priority of humans but its not useful now.
Keith Morrison mentions tanks, he's wrong about tanks.
German tanks were inferior to both Shermans and T-34s, the only thing that was good on them was the gun and ironically the German 88 flak gun out performed every German tank because it was also a superior gun.
German tanks were hand-built and hype. The hand-built custom build caused the tanks to be unserviceable. Their heavy armor meant they had a longer lifespan but that just meant they broke down in the field.
Russians knew their tanks wouldn't last more than 19 hours in combat so they didn't bother to build transmissions that would last longer than a minimum time, something like 20+ hours. So it's a misnomer that Russian tanks weren't well built. They had no reason to build certain parts better when battle attrition would make them explode.
Germans had great engineering, but their tanks died like all the rest.
Tigers never fought shermans on the western front, Fury is a myth.
Shermans were excellent tanks, well built, well machined, standardized parts so they could be repaired in field, and their armor was sufficient. The idea they were "death traps" has more to do with ALL tanks were death traps.
The attrition rates between Shermans and all German panzers was about 1:1 ratio.