Few reasons.
* T-bills are what's known as risk-free. Banks and credit unions are not, sure you have the FDIC/NCUA covering the first $250k but I'd rather not deal with it, especially since banks offering these higher rates tend to be financially weaker and struggling to attract deposits.
* I have not seen 6% much, other than a few institutions I've never heard of offering it on a promotional basis to get you in, usually it's in the 5.4-5.7% range, at which point it's no better than T-bills on after-tax basis since GA tax exemption is worth about 30bps once the yields are in high 5% range.
* Liquidity/access to capital. The higher rate CDs will have early termination penalties. With T-Bills you can schedule weekly purchases of smaller amounts so you don't have to wait the entire term to get your money back. Once your T-Bill portfolio is set up, functionally it's closer to a high yield savings account than to a CD when it comes to liquidity. There's also an option of transferring your T-bills to your brokerage account to sell them before maturity. Concessions and transaction fees should be minimal. Due to hassle I'd rather avoid it but it's good to know it's available.
Overall, banks are at a great disadvantage, courtesy of the Fed's aggressive pace of the fed funds hikes. Many are still sitting on huge loan portfolios made in the era of ultra low rates and they simply cannot afford to offer higher rates on deposits. At some point those loans will pay off or reset and the banks will be offering rates more attractive than the Treasury, but we're not there yet.
