A few things:
Firstly, it’s well within the remit of governments to choose to tax some things but not others. If they wish to tax income but not unrealised capital gains then they can. If you disagree with it then you can lobby against the tax law, vote for a party which wishes to tax differently, move to somewhere else with a different tax system etc. etc.
Second, what he’s arguing for is basically the same kind of thing as a wealth tax on the value of property, including both fully owned and mortgaged property.
Following his line of thought would have you agree that you shouldn’t be allowed to take out a mortgage to buy a property unless you’re prepared to pay a wealth tax on the value of it. Although most countries in the west have income taxes and capital gains taxes, the majority don’t have a wealth tax, which is basically what he wants.
Finally, there can actually be good reasons as to why a government would provide tax incentives for keeping your wealth in a company - it could be argued that it’s better for society if company owners are encouraged to increase the value of their companies by as much as possible and keep the wealth in that company, as it has knock-on effects for the wider economy and job creation. Taxing company value more punitively, or in a shared pool with your other income, could just encourage business owners such as Musk to extract their earnings from the company asap and pocket it, as opposed to keep growing and developing the business.
