Well now you're questioning the necessity of banks in general. Additionally, the government regulates them
into these securities.
>Why not just make the government do that directly?
If I understand, do you mean why not cut out the middle-man and have people buy the T-Bills/Bonds themselves? If so I completely agree, to some degree, that banks nowadays are nearly complete scams as far as warehousing your money, while providing near 0% interest, with the backdrop of 4 to 5+% risk free short term rates from the government as alternative.
I myself have taken out a bunch of cash and deposited into various tenors of treasuries at TreasuryDirect.gov
And many others have as well, which is why depending on which data you look at there has been a massive withdrawal of cash from commercial banks in recent months, with people either buying treasuries or putting the cash with their brokers (who are buying treasuries etc).
I don't know the answer to how this is resolved, or banks place in the economy... they are supposed to provide credit and money creation for business investment. Since the 2008 crisis, lending has been subdued... there are probably many causes for this. Banks have terrible apps, user interfaces, user experience moving money quickly (in the US) and terrible returns provided to you for lending them money (most people don't realize this is what you're doing when you deposit). So yea, they are a legacy, protected industry that scams their customers out of the spread between the treasury yields they are getting.