It's probably even worse than you think - it's not just about carrying a lot of weight. You're expected to carry it and still move pretty quickly on humps. In infantry in the USMC, going at 1-2 miles an hour with all this gear isn't unheard of, or 20 miles a day. If someone is faltering, it means one more person's gear that has to be split up for others to carry.

For some units, the amount of gear you have to carry can get bad, like carrying a 50 cal, or in the case of my unit, we had a lot of M240Bs to carry because that's what we had on our LAVs.