Except that God, for His own reasons, doesn't seem to work that way.
(Although Jesus did speak to crowds of thousands many times.)

Now, if a mere human thinks it ought to be done one way and the Creator of the universe wants to do it some other way, who do you think is probably "right"?

One possible answer:
God does not want to give us absolute proof. He gives us information through his prophets or shows up in person as Jesus and then leaves it to their associates to proliferate it so that there will always be room for those who don't want to hear to ignore it.

I'm sure if God wanted to he could put on quite a fireworks show every day at midnight. The fact that He doesn't do that tells us that He intends to leave room for doubt. This is, in fact, exactly what Jesus said:

Then the disciples came to Jesus and asked, “Why do You speak to the people in parables?” He replied, “The knowledge of the mysteries of the kingdom of heaven has been given to you, but not to them." - Matthew 13:11