Why won't facebook let me change my name?

it's not really that big of a deal, i just wanted to change it to something funny as a joke, but facebook won't let me! and i know that facebook has to approve all names, but a lot of my friends have like really weird names..like jenny t-rex squishybug, drinky mcpotatoes, casey switchblade pocketmouse..weird names that are obviously fake..so why won't facebook let me change my name to something like that?? it's bothering me lol