Why can’t I short Game Stop right now?

I can't know why your brokerage, specifically, disabled shorting, but the reason seems very obvious to me, like common sense.

Nobody knows how high the stock will go, especially not you. A month ago it was about $20, and it certainly wasn't ever going to be $50. But now it's $300. A 15x increase. Are you prepared for another 15x increase, where it goes up to $4500? Will you be able to cover that short?

And don't forget, the stated intention of this flash mob is to buy up all the shares so that no short seller can re-buy at all. It doesn't matter what the "real" value of the stock is, what matters is how much you have to pay to get one. You may end up obligated to buy a share from someone who's like "lol haha f**k you, gimme a million bucks or go f**k yourself". Got a million bucks to spare?

There are already rumours that short sellers won't be able to re-buy at all and that the reason various brokerages and clearinghouses stopped letting people buy the stock is to avoid their own bankruptcy from having to cover these defaulted shorts. (Meta: this section could be deleted if this is unfounded)

It is not a rare thing for shares to become hard to borrow or even non borrowable. Brokerage firms publish their hard-to-borrow lists daily.

I can't speak for all brokers but mine also lists the number of shares available for borrowing. It's not a good idea to short the stock if that number is low because the likelihood of getting a forced buy in is higher and that can be somewhat more painful with a stock like GME which has a wide B/A spread.

I don't know what Schwab's restrictions currently are (google for details) but as of yesterday they made GME non marginable (for use as collateral)and raised the margin requirement on GME stock and options.