Hey @josephgranata,
this is mostly due to security reasons to ensure that the Bitcoin transactions are eventually being confirmed by the Bitcoin network without any human interaction.
I kindly refer to this post of @THLO: ckBTC: a canister-issued bitcoin twin token on the IC, 1:1-backed by BTC - #199 by THLO
I agree that it would be awesome to be able to withdraw a lower amount. IMO we could also think about changing ckBTC fees for transfers and approvals. That is a totally different topic which I do not want to touch deeply at this point.