You half understood the problem. It's supposed to alert people to your presence... So making it sound like something that they would ignore, or a sound that suggests it is something other than a couple of tonnes of metal hurtling down the road is a really bad idea.
That's why there needs to be a standard for these things. Minimum volume level, some measure of how noticeable it is (e.g. no high pitch sounds older people can't hear) and something that suggests a car approaching. We already have similar standards for vehicle reversing sounds in the EU, for example.
The horn as well, it's considered a safety device and must conform to certain parameters so that it is actually useful. The Tesla ones, like the random guy talking for example, are completely useless for the actual purpose a horn is there for.