For the practical consideration of people asking about a date, as someone else said, I think it's usually the opening question about wedding preparation and/or otherwise continuing to show interest in the topic you, the newly-engaged, just brought up--your engagement. "Is there a ring?" and "How was the proposal done?" are another couple of common questions, IME. Kind of like, if you tell people you just bought a new car, they will show interest by asking you further questions about it--make and model, mileage if used, color if you're like me and know nothing of cars.
I think, mostly, they are really just saying, "We're excited and happy for you, and we want to know more."
There can also be a logistical component to it, as others have pointed out. People may also be asking, "Is this going to be very soon, like next weekend, or in roughly a year, or more like three years, or what?" Because, presumably, they're interested in attending the wedding and want to make sure they don't inadvertently book a big vacation or whatever at the same time. If people's attendance is something you're concerned about, it only makes sense to settle on a time frame, and soon thereafter an actual date, as soon as possible, so you can "book" that date on your guests' calendars. Especially if there are going to be issues like people coming in from other countries, which can take a long time to work out. So one answer might be, "No date yet, but we're thinking fall next year at the earliest, and maybe even later. We will definitely keep everyone informed, though."
On a philosophical level, I think engagements are very interesting. They can represent such different things to different people. For some, it's the start button on serious wedding and marriage planning. For others, it's the next level of commitment, but one that can have an indeterminate length. For example, one person I knew met her boyfriend in high school, and when they went off to different colleges, they became engaged as a way to reaffirm their commitment to each other. Although I'm sure many of those plans have gone wrong, in this case it had a happy ending, as they finally started planning the wedding during their senior year in college and held the wedding soon after they both graduated (and are still married today).
Another friend of mine was living with her boyfriend, serious relationship
, and he got a job offer that would take him across the country. She didn't want to have a long-distance relationship
and was prepared to follow him out there and continue living with him, but she was much relieved when they became engaged, so she would be accompanying him as a fiancee and not just a girlfriend--to her this seemed more stable and committed. I don't know when they started the wedding planning, but it probably took a bit because they had all the logistics of moving and her finding a job to deal with first. But they did eventually get married (and are still).
Like so many things, it's not the timeline that's rude, but how the couple handles it. For example, announcing that you're getting married next weekend and everyone MUST be there or you'll throw a fit? Rude. Being engaged for three years with no hint of a date, and then
announcing you're getting married next weekend, etc.? Also rude. Getting married next weekend but being totally cool about who can come or can't is polite, although it may still leave hurt feelings, which may or may not be reasonable.