It is necessary to build more housing stock, but if you simply do that alone while there is still significant incentive to buy investment properties then the developers will obviously sell to those that pay and it's typically those with means that will pay, which tends to be people who can afford multiple properties more than those who are struggling to afford one place to live in. Obviously if you're a developer looking for a return on your investment you'll price according to what those people will pay so that housing stock is quickly swallowed up mostly by landlords who will want to recoup their investment by charging higher rents and so on.

Since I started keeping up with all this locally, here is what has happened.

• About 20 years ago, there was a ring of realtors who would were actively manipulating the prices by buying and selling to themselves. They were caught eventually, but they had already caused the damage they set out to do, artificially hiking prices for their buildings.
• About 15 years ago, the government realized that people needs to take more loans for the economy to grow because when you get rid of taxes on the rich there isn't enough money pumped into the economy to make it grow. So they gave away thousands of social housing units to their friends (private entities) for free who then sold them back to the public.
• The landlords has since been running an "upgrading" scheme of apartments and houses all over the place with shitty things like a new fridge or a new and improved window or whatever have you, which allows them to increase the rent considerably outside of the limitations the law specifies. Of course this sounds like a bad idea because if you aren't renting to anyone you're losing money - but when the price of renting is 3 times the original price, it means they will still be profitable if they lose 1 in 3 renters. Not surprisingly, people have started to complain there is nowhere to live because of these conditions. The government isn't stepping in, because they need these people to buy properties rather than rent - forcing them to take loans, and bolstering the economy. It's why they started giving away apartments in the first place and causing this issue.
• As a response to demand though, and show of good fate, the government promises to build more houses. By giving contracts and money to private entities (their friends) to construct new homes. And to "fix the issue with cost", they promise a 20% discount on the price per unit to any buyers who invests into buying the apartment before it is finished construction. Naturally, the buyers of these houses and apartments immediately sell once the unit is finished, netting a 20%+ profit on their investment. Now suddenly there is a huge upswing of houses and apartments, none for rent of course, and average people can't afford to buy them. Investors though, they keep buying up everything, much of it is empty and not used, and sit on it for 3-5 years, and sell it to the next guy (or if they find the illusive average person who can suddenly afford it).

This is not to mention that the door was opened for american and chinese investors to buy up properties and land, shooting the average prices through the roof. Together with the government (of course a rightwing government has been in power for the past 20 years, causing all this to happen in their quest to privatize and let their friends milk tax money, reduce taxes on the rich, and balancing it out by forcing your average joe to take a massive housing loan), reducing the loans to just 3% to make them "affordable to average people", which further spikes prices upwards and it ain't coming down until something catastrophic happens.