1. Taxes are a hard problem. Besides basic W2s, There are a lot of deductions to know about, and it's hard to keep track of all of them.
2. Depreciation schedules. They vary from item to item, and there's a bit of a grey area on what you can use, and what you can't. What can have bonus depreciation?
3. Calculating cost basis is too damn hard for most of us. Not just stocks, but partnerships as well.
4. When can you take a distribution from a business? How many times is that money taxed?
5. Add another 50 use cases, one for each state.
6. How do the states interact for individuals with diverse activity? Easily 2^50 cases here, and my gut say more like 50!
7. Foreign activity. Good luck with that.
The bottom line is there still is enough complexity that this needs to be done manually. Not to mention there are a lot of things that are legitimate judgement calls, and vary from accountant to accountant (or auditor to auditor when dealing with the IRS).
So, that's why robot's aren't doing my taxes any time soon.