Indeed. I quite agree that in the majority of cases, it is neither sensible nor good manners.
But there are exceptions, which is what
@roubaixtuesday doesn't seem to accept.
I think there are a combination of reasons why speed limits don't apply to cyclists.
But the biggest reason, AFAIK, is that cyclists are not required to actually know how fast they are going. Motor vehicles generally have a requirement to have a speedometer (and the construction and use regulations require that it can never read low, but can read up to 10%+2.4mph high). Therefore, drivers of motor vehicles should always know what speed they are doing, and therefore whether they are exceeding the limit, while cyclists have no such requirement.
But then there is also the fact that until 20 limits started to appear, it would be quite rare for cyclists to actually exceed the limits anyhow, and when they do, the damage they are likely to cause in an accident is far less. They are also far more manoeuvrable, and have a much smaller frontal area to colide with others.