For a large number of people, the likelihood of the us government stealing all of coinbase's money is far lower than the likelihood of them losing their key to their hardware wallet or whatever.
Let's just store all of our money with the government then, eh?
With a bank, yeah. Even as somebody who is savvy specifically in security I trust a bank to keep my savings safe more than I trust myself.
Threat models and risk balancing are important.
Banks are basically just retail fronts for the Federal Reserve anyway. And IRS has such tight hooks into the banking system...all they have to do is snap their fingers to freeze your account.