The minimum retrieval amount was reduced from 100,000 to 50,000 sats not too long ago.
Since ckBTC is burned first and never reimbursed, the ckBTC minter’s Bitcoin transactions must go through eventually. A relatively high minimum retrieval amount ensures that the replace-by-fee mechanism can be used, even repeatedly, continuously increasing the fee (paid by the user) until the transaction is mined in a block.
In short, the minimum retrieval amount is kept relatively large for security reasons as it increases the chances that a transaction can be pushed through and the ckBTC minter gets the change back.