The minimum retrieval amount could be changed through a proposal with the corresponding code change.
However, there is a reason why it is so high: When a retrieval request is processed, the desired amount of ckBTC is burned first. This operation is irreversible, so the same amount in BTC (minus fee) must be released to the user. It is possible that fees go up so much that the transaction will sit idly in the mempool and is possibly dropped (leaving the user with neither ckBTC nor BTC).
The ckBTC minter can handle this situation: It (periodically) re-submits the transaction with an increased fee, which means that the user gets less because the user pays for the fees.
This is only possible if more can be deducted from the amount that is paid out—ideally leaving a big chunk for the user. The minimum retrieval amount of 1 mBTC ensures that it is very likely that a transaction will eventually be mined in a block, even if the network fee increases, and that the user still gets some BTC as a result.
In other words, the minimum retrieval amount could be reduced but that would increase the risk that a user cannot get any BTC out at high fee rates.
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