They aren't looking to shoot at soldiers with these things, they are for taking out vehicles and equipment.

Anti-materiel is it's primary purpose. For that 14.5mm beats the .338. The pictured weapons are for penetrating engine blocks that are stationary or moving with simple motion, within a greater effective range, not body armour (but will all the same). Long range extreme precision is secondary but is still a feature of 50cal and 14.5mm (top 3 records sniper kills are with a 50cal/14.5mm: #3 is actually with one of those type rifles).

I understood some of those words

Maybe it's just easily accessible ammo.

Yeah, other calibers are available, but this might be the most abundant around.

You forget it's not a precision rifle, and it's not meant to be used against Infantry directly. It's accuracy isnt meant to be measured in minute of angle but instead minute of APC