Why is bad writing so common, though? I think this needs further exploration. What are the incentives of the different actors? TL;DR CYA

Clearly, the reader just wants to get the information that is relevant for them personally, with the least cognitive effort. Then move on with their day or to the next step in their workflow.

The company/government can have various incentives on various levels of the hierarchy/bureaucracy.

The boss/owner may want the company to communicate in a snappy, hip, youthful way, in a friendly and clear tone with no bullshit.

However, the person/department actually writing the text may want to deflect responsibility, avoid possibilities for blame, keep things vague enough so the onus is on the customer/citizen to make sure everything will work out. They don't want to deviate from process. The customers/citizens should be uniform with no exceptions, because exceptions cause work. If you write too friendly, people will assume they can get favors, that we can start bargaining because they are talking to a real person. Text written in a cold, dispassionate, bureaucratic style will not invite such answers. It signals that things are set in stone, that there is a process in place and we won't say A or B, they will have to figure our A vs. B themselves.

For example, I'm a teaching assistant at a university. If I'm buried under too much work with research and get lots of complicated questions from students, I give vague answers so I can't get blamed ("But XY said it would work out this way!"). Also if I use a bunch of official words and formal sentence structure, the student gets the message that I won't do the work instead of them: there can be tons of different rules interacting, different study programs have different rule books etc. In the end it's their responsibility. I cannot give definitive answers, but experience shows, that writing things plainly often invites more haggling than writing in an official tone.

The same thing happens when people talk about medical, legal and other potentially dangerous topics. Even if the actual answer is straightforward and will apply in 99% of the cases, nobody gets fired for being too cautious. "Nah don't do that" is the easiest thing to say. Ask your doctor. Ask an attorney. Check the laws of your region.

To write plain and clear text, you need to leave all these caveats out. But people will jump at every opportunity to try to shift the blame, and if you weren't careful, it will accumulate on your desk. To avoid this, orgs and lawmakers should make sure to reduce the legal responsibility for saying things.

For example I know a(n underpaid) lawyer working for a governmental institution in Hungary. They are in the communications department and give advice on certain legal things to ordinary citizens phoning in or over email. Fortunately, the laws and rules are set up such that they cannot be blamed for answering wrong or not hedging enough, they can write as they would to a family member. It's not "legal advice", because you can't know all circumstances from just an email question. At best, the person or company who acts on such advice can use the email exchange to demonstrate due diligence. They at least tried. Still, it's fully their responsibility to check and interpret the law.

Therefore, unless you shield your writers/departments from blame by similar explicit rules, they will shield themselves through vaguery and hedging and complicated writing.