To give you a small example: the deduction for moving expenses[0]. If you move for work (plus some other small conditions), you can deduct those expenses from your taxable income. This can't be claimed for you automatically, you have to ask for it. To automatically file your taxes would very likely mean you miss this deduction. It's the legions of (arguably) boutique tax credits like this that crank up the complexity of filling. Individually most of them are reasonable, but it would be hard to take them away because they are likely to be used by a lot of people (the mortgage interest deduction for homeowners, for example).
We have those - but the system is constructed to be simple. If a not insignificant part of the population is eligible for some deduction, then either a) ensure it's automatically registered or b) remove the deduction or raise tresholds until everyone can again file automatically.
For example, interest rates, pensions, charities, home improvements etc are deductible and automatically deducted since e.g the bank or charity reports the interest cost or donation etc. Travel to work is deductible but the treshold for making the deduction is so high that few can claim it.
The key is to design the tax system so that it is easy to file. A deduction no matter how fair and reasonable that would make thousands have to file paperwork doesn't make sense.
But you are overlooking the fundamental underlying problem. You didn't mention whether the "easy to file" tax system would be an advantage to Democrats or Republicans. Without knowing that, it is impossible to assess any piece of legislation.
Wouldn't it be a benefit for most taxpayers, and only a drawback to a small minority of taxpayers and companies involved in e.g tax law or tax software?
If a law is merely about bureaucracy and is neutral in terms of e.g tax pressure of redistribution then any political resistance must surely be because of some kind of unwanted influence from an organization or company?
Things like this exist in Sweden, and they aren't a problem.
When you sign on to the government tax website (or mobile app, or whatever) you are presented with all the numbers they have for you, which means whatever your employer(s) and bank(s) have reported on you. (Same as in the US, the IRS knows these things already, they just don't tell you what they know).
If you are fine with that, you sign it, and it's done. Denmark has their system setup so that if you do nothing, it's the same as accepting.
But if you have more information, if you have deductions to make, and there's plenty of those in Sweden as well, you just fill in the information in the very nice government tax app, and you're done.
0 - https://www.irs.gov/taxtopics/tc455.html