Because the money used for the investment was stolen.
Could Anthropic now say, "Oh no, that money was stolen, the investment was never valid, we're returning every last cent." and essentially do a huge share buyback at a steep discount to current valuation?
A liquidator will evaluate the assets current market value and try to liquidate them in the most advantageous way for creditors.
Which means: they'll try to get at least the valuation that Anthropic got from the AWS deal. The problem is: they need to find someone willing to buy these shares at this price. If they do so, FTX creditors will be happy.
And it's up, so if you think they should renege investments, start with the ones that failed and not the profitable ones.
Clawbacks would make sense for a charitable donation or shares that have gone down in value. For an investment that's increasing in value, it makes more sense to sell it at market.
Because of its investment, FTX owns shares in Anthropic. In bankruptcy, those shares will be sold in some form to new investors in exchange for cash, at whatever the current value is. Just like any other asset.
In theory, I suppose that new investor could be Anthropic itself doing a stock buyback, but that would be extremely unusual. Buybacks are for mature companies with excess cash, not growing startups.