Because they can't be arsed to go to gp and collect from the pharmacy?
My son has eczema and we use aveeno for it, we used to collect from pharmacy but go through it quite quickly.. At least once a month.
Also his bath oil etc I buy online now.
This is because when we used to collect it, it took ages (had to wait at least 30 min and queue) and was such a pain in the arse.
Also the obligitory gp appointment every few months to prove my son does indeed still have eczema. 
To me it's worth the cost even though otherwise we would get it for free.