Its called 'vesting' like investing and being invested. The whole point of SP is to have people vested in Steem, and have their money tied in for a fixed period of time - like a fixed investment at a bank. In return for this, SP offers extra bonuses such as higher upvotes, and the ability to be delegated to other users and accounts.
If people could take out their SP at any time there would be no stability and I think that the price of Steem would become alot more volatile
Firstly for security purposes. I for one are happy that it takes 13 weeks to fully power down. If you could just withdraw everything it leaves the accounts at a huge risk of being hacked. this also prevents flooding the markets with steem and keeps the prices more stable.
If people would be able to do that, the whole Steem system would be empty by now. Simply because, if you have a drop in value, as is happening right now, a lot of people become bearish, and start selling their crypto's. What is basically "powering down" in Steem. By having a retention period of 3 months, Steem is buffering this, and keeping its userbase and tokenbase more active.
Because if you allow people to just pull all their steem power then you would create massive market pumps and dumps and people would abuse the system by running the power out of one account then swaping it and draining the next one.