You are conflating a number of issues here.
Why does my software have all these features to begin with?
Because other computers' software has those features, and network effects punish any software developer who doesn't follow the herd.
Let's take an example from your question: Why does my web browser need to do anything other than basic HTML and CSS? Well, have you ever tried browsing the modern internet with all JavaScript disabled? It's functionally unusable. The problem is that, once JavaScript existed in a widely used browser (Netscape), people started using it in their webpages. That meant that other browsers had to add support for it to prevent their users from complaining that webpages are broken. And once more users had support for it in their browsers, more webpage authors started assuming that users had it. Round and round the positive feedback loop goes.
Software developers have strong incentives for adding features to software, and strong disincentives for removing them or even changing them.
It's taking 25 years to kill Flash Player, despite it being an incredibly complicated, bug-ridden, security nightmare black box that Adobe themselves no longer wants anything to do with. You know that all hell is going to break loose on December 31, 2020 when the plug finally gets pulled and people can no longer play their beloved Flash games from 2001.
Why do employees of a company need to have full PCs rather than dumb terminals that can only view intranet pages?
Because intranet pages work great until an employee needs to do anything other than the specific tasks that the programmer has predetermined are that person's job.
What happens when someone else (either inside or outside the company—let's call her "<PERSON>") wants to send you a presentation to review and edit? Every person at every job role at my company has had to do that at some point—managers, engineers, administration, facilities, you name it.
You can receive <PERSON>'s presentation file on your internal webmail, but you need some way to edit it. And that means your computer needs to be able to edit PowerPoint files, because that's what everyone uses. 81
And PowerPoint files are ludicrously flexible in what sort of content can be contained in them. So we're back to the network effects problem. If <PERSON> sends me a PowerPoint file and I can't edit on my computer because it uses features that my software doesn't support, that's my problem as far as <PERSON> is concerned.
It's functionally impossible to forsee all the things that someone might need to do for most modern jobs, for the simple reason that if it were possible, that job would probably already be fully automated. And that's not even accounting for the fact that many companies allow their employees to use company computers for all sorts of things that are not strictly part of their job description, such as streaming music.
There's also a more general principle here. As you correctly point out, most PCs are "fundamentally overpowered and overcomplicated for the vast majority of tasks that any person employed by a normal, non-highly specialized IT company, or government entity, would logically need." The problem is that a computer that does all the tasks you need 95% of the time but is useless the remaining 5% of the time when you need to do something weird and specialized—is useless as a computer.
I've heard it said that in software design, 10% of the features cover 90% of what any user needs, but that the remaining 10% of what any user needs is different for each user. If you take the set of features used by each user and intersect all of these sets, the result is not sufficient for any user.
"This interface would have a simple username/password system, with no demands to reset passwords or 'two-factor auth' or any of that nonsense"
This gets its own subheading, because this is a totally separate class of issues from everything else you mention. Go over to Security.SE and read about why these things exist. There are very specific reasons for these security practices, that are totally orthogonal to any discussion about complexity of software. 81