Never mind how ridiculous your question is to begin with (what do landlords, the useless money syphoning middlepeople, have to do with building???), but the reality is that there are already enough empty properties to house all of the homeless people in most countries you check (US, UK, Canada for starters), not only once, but several times over.
It isn't lack of housing that causes homelessness, it's capitalism and the selfish greed it encourages.