Given the number of times people assert this, I think Twitter must have explored this option and decided it wasn't a viable option. I don't know why, but I'll assume that they're smart enough to have decided against it for a good reason.
I would not make the assumption. I've been working for corporate America long enough to know that there's an astonishing level of incompetence many places. I'm sure Twitter is no exception. I'm sure _someone_ at Twitter has considered this, but perhaps they work for someone who doesn't "get it." Among many other possibilities.