Because of the massive investments into datacenters. Why is everyone so quick to drink the kool-aid. They are spending stupid amounts of money to capture the market and force everyone into a subscription service, not because it cost money to actually run the models. They are all switching their models to fine tuned quants after the first two weeks as well.
The open source community has found dozens of ways to lower costs, do you really think these big companies aren't using the same tricks and haven't developed even more advanced techniques?