Because government is an easy way to work together on big problems. Things like charity are great, but the participation rate tends to be a whole lot lower than you can get from taxes.
The OP was praising high taxes. If he paid lower tax, he would have more money to spend, also on helping others. Do you think it's ethical to force people into charity?
Simple. I trust the British government to do the right thing more than I trust the likes of HSBC, Microsoft, Google, Unilever and any other company that's built by greedy people whose sole concern is generating profits for themselves and their shareholders.
Wait. Do you think it's the right thing now for the FED, ECB and other central banks to print money day and night and eradicate savings of hard working people via inflation?
I'm not sure why you bring up those companies in response to my proposal of managing your own money. But since you mention how purely evil those companies are, last time I checked they were employing real people, working on real products and services, supporting their families and contributing to the economy. I'm not a fan of big corp, bit it's not so black and white.
I didn't use the word evil so I'm not sure where that's coming from. I pay less than 45% of my income in tax to the government. With the rest I invest a large chunk in the private sector anyway. So I know it's not black and white. But in times like these I'm glad I invest in the government.
The government is already doing that for me. My tax contributions go towards pension, healthcare, public services... I don't have to worry about taking a bank loan to go to the hospital or what I'm going to do when I retire.
I used to take things for granted, now news like "I have to pay $30k for Corona virus hospital treatment" make me appreciate being born in "socialist" Europe. To each their own.
Do you think the government is managing your money better than you would? Aren't you concerned that big part of it funds the bureaucratic apparatus itself? Aren't you concerned that you are not paying for your own retirement and in our aging Europe 30 years from now there will be not enough working people to pay for yours? Don't you think that paying a lower tax (e.g. 25%) would leave you with more money to cover that 30keur for yourself or generously help others?