I think sometimes people may be a bit confused by the signs. Like for example some literature says that the signs are night waking, steady weight gain, being able to sit up by themselves, watching you eat and grabbing out for food. Some of these can easily be confused with natural baby development. Like for instance, it is natural for babies of 4-6 months old to start practicing picking things up and exploring objects with their mouth so when a baby sees us eating they instinctively want to pick it up, grab it and put it into their mouths. I have also spoke to some friends who weaned their babies before the 6 months because of night waking but then when they introduced solids this didnt alter their sleeping patterns and night waking continued. Sometimes people have weaned their babies because of an increase intake of milk and they think that the milk isnt satisfying their baby when if you look at the age it usually coincides with teh 4 or 5 month growth spurt so and increase in milk is to be expected.
I also think there may be some pressure from other people especially older people who are so used to weaning their own children and possibly grandchildren at 3 and 4 months that it is the norm to them and they only know what they did for their children.