The reason that tax is lower on dividends and capital gains is that both are signed of a successful business.
Dividends are the distributions of profits from a company and aren't a tax deductible expense in that company, so the theory is that to have made a profit that can be distributed to its shareholders it must pay tax on it first.
So while an investor might only pay 39.35% on the dividends, the company paying the dividend will, if UK based, have paid 19% corporation tax on its profits (this is going up to 25% for larger businesses next month).
So, for example, a company makes £250,000 profit. It pays £47,500 in corporation tax and distributes the remaining £202,500 as a dividend. If the shareholder is an upper rate tax payer, they'll pay £79,684 in tax.
HMRC will have received, from a £250,000 profit £127,184 in tax (50.9%).
Similarly, if someone has disposed of assets at a profit (and let's face it - all Rishi's come from investment funds, not property), then it's again a sign of a successful business as share prices generally only increase if a company is doing well. Those companies will have been paying tax, so the capital gains tax is an additional tax take.
For anyone interested, the full document is here - Government link
Based on his 2021/22 figures, he will have paid £7,002 in NI on his Employment income, which isn't mentioned on the letter.
In 2020/21 this would have been £6,986 and in 2019/20 £5,939