The reason for that is rooted in the security mechanism of staking - if stakers are able to unbond immediately, they can perform a so-called front-running attack on a potential slash penalty and therefore compromise security.
You can also use the RAGE LEAVE option: This means you can unbond some of your staked ADX tokens (or all) without waiting 20 days BUT please note you would only receive 70% of the withdraw amount. The remaining 30% will be distributed amongst other stakers.