Technically at the point of Prometheus humans didn't know about alien life as far as we know and had no reason to suspect they would need more protection than usual.
You can't assume the future means a more Utopian and relatively smart society. Or that the Weyland Corporation and industry in general would have a driving want or need to engineer better safety gear. This is a rich and greedy corporation after all. There's no telling how many man hours and how much money they exhausted making androids instead of making humans safer.
In Prometheus, the humans were glad for the chance to go on the mission or were just getting paid to do a job without said safety gear. They were only useful for what information they could provide about the host planets, moons, susceptibility, etc. up until the point they were expendable. Even after alien life was discovered in Prometheus, Weyland Corporation later showed they were willing to sacrifice the human crew just to have Ash bring back a Xenomorph.
It really isn't a stretch considering what technology has brought us since the 20th century. Are we any safer than we were in the 1900's? You could argue we have longer life spans but that's mostly due to drugs that prolong death from conditions we largely inflict on ourselves. As technology progresses so do the dangers it presents in civilian life or otherwise.
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