Why do they do it?

London to New York in 90 minutes: is this the Concorde of the future? - Science, News - The Independent

Why is it that, about every 12 - 24 months, there is always some airline manufacturer, aerospace company, or university research group, that comes out with some vision for a super-super sonic (OK, hypersonic) jet rocket thingee of the future that will get people across the world in the space of a few hours?

No one seems to consider that these are at all very likely with current or near term technology, do they? This latest one, linked above, says that one stage of the process will involve liquid oxygen/hydrogen rockets, just like that remarkably (un) reliable space shuttle did.

As much as I like high technology, I find these announcements vaguely ludicrous.

For the moment, I would be much more impressed if they were devoting much more time to planes that have as high a fuel efficiency as you can achieve, even if they are a bit slower than current ones.