Best guess: The back end processing is actually performed by humans at the IRS office in DC and the form is simply an online "request" to one of those humans. And the office where those humans work is only staffed from 7am to 10pm EST.
My guess is it’s some ancient mainframe or even pre-mainframe app that is running on batch jobs that still require production control overseen by humans.
Federal budgets aren’t like private sector. They don’t just allocate $2B for the IRS commissioner to use as best suits the business… specific programs and tasks are authorized by congress. In the case of IRS, there’s a group of people in congress who are dedicated to IRS performing as inefficiently and ineffectively as possible. (For some things, earned income credit enforcement is fully modernized and effective)