Look at traffic jams, if what you claimed was true, then traffic jams in LA should never happen.
Building homes and all the supporting infrastructure from roads, schools or even parks takes a lot of time and money. The movement of tourists for example is always going to be significantly quicker, because its very easy to move a bus of people to Madrid, while it takes years to build a home for them.
I understand everyone has an opinion about housing prices, but your intuitions may not be well adjusted to Spain's circumstances.
Secondly its very easy for people to move, while homes are fixed to the ground.
These are just too examples why in this modern age of constant change, there will always be a housing crisis. Looking at the world, its not only Spain with a housing crisis - its basically every single country and the cause is "change".
THis "change" and movement of people means the building of homes can never win, because it takes years to build a home and the supporting community of schools and hospitals.
According to your argument, the answer to when to build is a good moment to build is "never". I can't agree, and there are counter examples too. The mass rural to urban migration happened less than 60 years ago, and the answer wasn't "do nothing".
You are focusing on just one input of the many that happen in a country. Spain has high levels of immigration, other countries have other forms of people movement for different reasons.
Today is also very easy for people to move vast distances, from across town to new countries.
Building new homes and communities where people can live takes years, people can move anywhere in days. The building is slow, movement is fast, the two are out of balance.