Loans against stock are not income because the gains haven't been realized yet. There's still a risk the stock can drop so he hasn't actually earned anything. This is exactly what happened with Tesla and Twitter.

Elon took out loans against Tesla stock to buy Twitter. Tesla stock fell. Then he mismanaged Twitter (X, lol) and its value fell (theoretically because it's private). Elon is not a smart man. Not only is he bad at running a business, he's also bad at managing his money. He's spending (actually wasting) money that he hasn't even earned yet.

It's like someone taking out a home equity loan. They wouldn't get taxed on it because it's a loan. They haven't sold the house yet.

Although there is a good argument for higher corporate taxes. That would tax the money before it became dividends or buybacks or capital gains or a margin loan (what you're talking about). Corporate taxes are simple and hard to avoid, if created properly. Just make treaties with other countries to avoid the dumb "Ireland" tax scenario where Apple designs phones in California, builds them in China, sells them around the world, and pretends the company actually exists in Ireland.