You just got your answer: Because it is agreed that it helps all of us.

Don’t want to pay taxes? Stop drinking tap water. Don’t turn on your lights. Stop driving on the roads. Don’t seek any medical care ever. Don’t catch the bus. Don’t even look at a public park.

If everybody pays a fraction of their income, these things can be funded. If we all went it alone, do you have the money or skills to maintain the roads you drive on? If you don’t, do you think the other guy is going to do it for free? Or cheap, knowing that he has skills you don’t?

EDIT: it can be argued about the proper use of tax funds in any particular country, eg an excessive ‘defence’ budget in the US.