Because Altman is eying IPO, and controlling the valuation narrative.
It's a bit like keeping rents high and apartments empty to build average rents while hiding the vacancy rate to project a good multiple (and avoid rent control from user-facing businesses).
They'll never earn or borrow enough for their current spend; it has to come from equity sales.
with very particular exceptions at the high end (like those 8-figure $ apartments by Central Park that are little more than international money laundering schemes) this doesn't really happen irl
an example: $5000/mo apartment generates $60,000 a year; forgoing one month of rent means you have to now generate $60,000 of revenue in 11 months, which in a bad market will likely not rent for $5450 if it didn't rent for $5000. Your mortgage still continues to pile up along with insurance and taxes, so you can't escape the hole.