Leverage. Deposits used to be bigger and amount lended used to be a smaller multiple of salary.
In recent times, deposits have been 5% (or 0% if guaranteed by a parent) and leverage upto 4.5x salary.
When you increase the amount of leverage available to people bidding in a supply-restricted market, many will max out that leverage in order to win the bid.
Add schemes like LISA, HTB, shared ownership. Cash-rich buyers, foreign buyers (recent BBC article on Hong Kongers coming here mentioned the ones using an intermediary firm having an average £700k wealth for example).
People being gifted or inheriting money from parents.