The OP asks for a 'proper' answer to the situation posed and indicates that only a few of the posts have tried to do that, preferring instead to contemplate their own insecurities that come to the surface when someone moors close by, just when you had hoped for loneliness. (Leave aside a debate on the difference between being able to choose loneliness and those who have it as an enforced permanent condition) In the interests of disclosure: our first reaction in such situations is invariably negative but it soon passes! Having a degree of hearing loss means, for me, that most annoyances can be ignored by removing hearing aids!
A few posts have posited reasons (like security) and here is another: many boaters on unfamiliar waters will use published guides to plan ahead where they hope to moor overnight and, on some canals, there can be a paucity of places marked. Sometimes we approach a planned mooring in some trepidation, knowing that if it is full then we have a lengthy and uncertain unplanned extension to the day. What most guides fail to mention (and it is not an exact science anyway) is just how many spaces there are at one of the marked moorings or even what the depth is like. Some new boaters may also be unduly influenced by the often strongly worded cries (here and elsewhere) for boats to moor closely on busy mooring spots. (Share a Ring) It is possible that some have done so in the belief that this is the right thing to do. It is always worth remembering when criticising the behaviours of other boaters, that changing them may have unintended consequences!