That's a great question in my opinion and the answer lies in the ways money moves around.
There are a lot of ways to get money that are not limited by income tax, depending on your specific country and tax structure.
Examples can be rent, inheritance, gifts and revenue/profits. I'm addition there is all the money that was earned elsewhere.
I'm short the sales tax is aimed at all consuming entities while the income tax is limited to, well, income. To give another example: Companies themselves consume but don't give any income tax.
Now the follow up question might be why it's designed that way. And that is something for the historians to answer. Make the sales tax too high though and people will country hop to shop, make income tax higher instead and you increase wealth gap even further (non income earnings are rewarded more)
From a tax perspective:
Stock earnings are not income.
Neither is capital gain - and it would need to be tackled as you suggested with q capital gains tax. But that is quite easy to circumvent in basically all countries because it's nearly impossible to check (as seen in the many scandals like cumex).
And then there is the whole banking apparatus: I can use stocks as security against cheap loans, use the loaned money for consumption and then lower my capital gains tax by calculating those loans against my gains. Remove that loophole and suddenly a pensioner can't pay for their running costs anymore
All of that lead down a rabbit hole of "but can't we just" for me. At least my conclusion: I didn't come up with something that a) doesn't break apart already on paper b) can be implemented without international collaboration and c) can't be abused just as easy.
Personal note and speculation: often I read people taking about a system that is designed against group X. I disagree: the financial systems were not designed with any goal in kind at all outside the specific needs and questions for the specific legislatures. Meaning that "we need to fund a social security system" and "why don't billionaires pay more tax" have the same root: we humans are terrible at overseeing time horizons of more than a few years. But we're absolutely genius in taking something apart and finding loop holes the size of a raisin.