Because 1) PHP syntax for object literals is non-existent 2) JSON is used in many more places than just Javascript and 3) the syntax of JSON makes sense; it's compact, human readable and easy to write. I'm not suggesting copying it 1:1 verbatim, but something close to JSON would definitely be an improvement. See for example how MongoDB uses a JSON-like syntax to great effect.

If for no other reason than ':' is easier to type than '=>'?

Hell will surely descend upon me, but I'd vote for having it be syntactic sugar for => in general.

You can already use the js-style shorthand inside of Twig templates, and I've been caught absentmindedly forgetting to switch to the 'proper' way for raw php now and then. And I would like to never need to even touch the shift key when having to write out an array or an object but even JSON won't let you do that. It's a step in the right direction, though.