[coryfklein]

Same reason you can reasonably refer to a male-looking acquaintance as "he" without enforcing a policy of using "they" until you have the opportunity to ask what gender they identify with.

Yes, you will get it wrong ~0.1% of the time, but the cost of slight embarrassment one one/both sides is far less than the cost of using non-standard and confusing communication the remaining 99.9% of the time.