You are absolutely right some of them are chosen by the builders ( brown envelope comes to mind) some purchase contract may stipulate that a particular notary has to be used. The reason given by the developers
a) As notary do/may not agree with others it creates problems.
b) Its will be cheaper as the notary will have all the papers.
c) We have been dealing with this notary.
I am aware that a notary friend of mine in france has confirmed ( a & b)
but I am afraid as we are talking out about Spain, I am not sure if I can give them the benefit of the doubt. sorry to all the legal people in Spain for my views but I think the system in Spain has forced these kind of view onto themselves.