Even if we ignore the financial cost, we are nowhere near being technically able to build a self-contained environment. We have never done it in Earth's orbit, or in the Moon, or in Earth.

So the problem with sending people to Mars is that it is not just a huge one time money sink. It is a permanent money sink, that would have to be constantly re-supplied.

People didn't just get in a boat one day and suddenly discovered America. They had been able to survive in a boat for months or years for a long time before they even got there. Nobody is saying that we should never get to Mars, just that there are previous technological goals that we are nowhere near solving yet.

Solve those goals one by one, and at some point you'll be able to get to Mars. But don't expect to stuff some people in a ship with a few lettuces and some water and send them to Mars without having worked out first all the boring little details of what such a travel would actually entail.