The question is... why do baby food companies seem to think babies need food from 4 months old?
Because despite the allergy correlation studies and WHO recommendations (which i believe in this specific case were based on expert opinion rather than systematic evidence-based study review, which is in fact against the WHO's OWN recommendations on how OTHERS should make recommendations) there actually hasn't been overwhelming evidence that giving babies in developed countries solids before six months causes problems in the vast majority. For the few babies with allergies in their families not giving solids MIGHT help reduce their risk factors, but not for all - it isn't direct cause and effect. The recommendation was actually aimed more at the undeveloped world where the water supply and poor quality nutrition means younger babies are at serious risk of illness and/or death the younger they are given food that is not breastmilk. The last study i saw (done in 2000 i think) found that babies in North Africa were on average breastfed for 19 months and offered solids from 4.3months.

There is always a cost benefit balance with government and big business. Those governments which have decided to encourage mothers to follow the BF to six months recommendation have mainly forced FF companies to cease advertising their infant formula (but NOT the follow on formula) to this aim, but big business pay ALOT of taxes, and the government won't anger those companies when they don't have to. If mothers know the WHO recommendation they will not offer solids before 6 months, thus they will not force the companies to change their packaging.

It also comes down to accountability and responsibility. So far NO-ONE has managed to win a court case against a baby food company for harm caused to an infant by being fed solids before 6 months of age. The link isn't strong enough to stand up in court.