Sorry Honeylove, didn't see your post there!
Please keep in mind that this BLW post is about feeding yes but not feeding purees and or baby cereals, in the world of BLW spoon feeding (where the baby isn't 100% in control of the spoon) is a no no. BLW is about following your babies own queues for eating and starting solids.
That said, I will gladly answer your question.
6 months is a guide I personally think. It would be the time when most babies have sufficiant control of their limbs and neck control to be able to feed themselves. It is also said that at 6 months the tummy is mature enough to handle food. The guide for BLW states that your baby is ready for food when he/she can sit supported, pick up food in their first and take it swiftly and accurately to their mouth and make chewing movements with their mouth. Most babies from 4 months on are unable to do this.
Until 2003 the food companies (like purity/gerber in USA/UK) in my opinion dominated the market and 'scared' people into believing that a baby needs to eat purees (i mean who wouldn't if your cashflow depended on it!) before 6 months of age because until then everyone agreed that 6 months was the age to introduce 'finger foods'. In 2003 the UK actually announced that babies should be exclusively breastfed until 6 months of age, but they never forced the food companies to change the writing on their packaging.
I think BLW and it's principals isn't as popular as it should be because it's a lot more work in the cleaning and healthy cooking department. Lets face it, it's easier to just have a stash of purity pots. And it's a difficult thing for women who have control issues (

) because BLW is completely in the hands of your child and a lot of mothers can't stand not knowing how many cubes/bottles of purees were consumed.
I will tell you this though, my pead said at 3.5 months that I can start feeding my LG and I said to him goodbye and have never been to a pead again. When it comes to feeding my child I'd much rather take the advice of a nutritionist (or someone who specializes in food/feeding) over a pead.
Oh yes and to just touch your point on milk not being enough for some babies, yes it could be the case. I started at 5.5 months because my LG was ready, perhaps there are babies that are ready sooner to feed themselves but when it comes to milk feeds we would like our children to be in a routine (like every 4 hours) and as soon as they need more or more often we run for the baby food isle, that is why BLW encourages demand feeding until 1 year of age.
Food is fun until one is the motto. And anyway, "Babies grow more in their first year than at any other time in their lives and they need nutrient- and calorie-rich breastmilk or formula to do this; solid foods - whatever they are - don't contain anywhere near as much nourishment."