Stocks aren't complex. Your broker has all the information the government needs. In the US, you have to manually enter that data into government forms.
Sweden here again. Most of our stock brokers submit our taxes from trades to our tax authority. If you're doing basic buying and selling, it's all prefilled in tax form every year.
This is just not true. While that is more complicated, it's still stupidly complicated to do something as simple as fill out the W-2 form if you have a single job with a salary, and if you're a freelancer with lots of clients that don't pay you the same way and don't give you a salary it's all but impossible.
This is somewhat hyperbolic. Reporting income to the IRS is pretty easy. Dealing with deductions and credits is not hard, but is hard to get right. The whole process ought to be much, much easier than it currently is, but it is not "all but impossible". I even managed to teach all 3 of my kids how to handle 1099s and other stuff, and if they were a bit intimidated (they were), they didn't find it actually hard.
I don't believe I'm being hyperbolic. Maybe everyone else is just smarter than me, but every time I filled it out myself it was extremely confusing and difficult to the point where I gave up and was generally told I'd made tons of mistakes when someone fixed it for me.