Because you can't. You have to pay, or according to your carrier provider policies, you can ask for an unlocking code if you subscribed to their services since 6 or idk how many months !
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Phone : LG P-700 running Carbon with CM11 kernel × Cr3pt
Tablet : Asus Nexus 7 2013 stock rooted with Glitch Kernel.
I've unlocked my 3g modem and 2 phones in the past with DC Unlocker, It's a lot cheaper than unlocking via your telco.
Be weary of sites offering to unlock phones though, the majority are scams.