It's really the same as English, in a way. Where Swedish uses just one word kommer, English needs two ("does get"). If we reduce it to one word in English as well, it would be "how gets one to the beach?", which although in practice a fairly unusable phrase mimics exactly how Swedish does it.
And likewise, if we used two words in Swedish - e.g. by introducing a modal such as skulle komma ("would come") - it would show the same structure as the two-word English version does: hur skulle man komma till stranden?
Look at it this way - in the English "How does one get/walk/drive to the beach?", each option means something completely different. Their Swedish equivalents are usually close, it's just that we use kommer to mean "get" here. And come to think of it, I wouldn't consider "get" much more logical. :)
We do accept both "how do you get" and "how do I get" as well, since they're both perfectly idiomatic translations.
The reason for "how does one get" being the default is that whatever we put as a default is automatically chosen for the reverse exercise, "translate into Swedish". In other words, if we put the "you" or "I" versions as the default, you'll never be asked to translate from English into man - which is a very common and important construction to know in Swedish.
Hence, we sacrifice a little English idiomatics for the benefit of being able to better teach Swedish. :)
"One" as used here is a bit more formal and more British English than American English. I always think of it as the Queen (or a royal) saying something like "How could ONE possible do that?" as part of the 'royal we.' I looked up "One is not amused" when I started Duolingo and read that it is credited to Queen Victoria but inaccurately, as she would have said "We are not amused"...the 'royal we.' It helps me remember the Swedish construction (with a smile).