Most people seem to find reading much easier than writing. They'll read lots of emails, books, articles or blogs, but rarely write replies. They think writing is a big burden.
The reason reading when tired can be hard for me is because I have to remember a bunch of different things they say, figure out what they mean, and put them together to understand the bigger picture. That requires loading a bunch of foreign stuff into my memory. It requires translation work to get from their ideas to ideas that make sense to me.
When I'm tired, I can read short things one at a time. I can read individual paragraphs and reply to them fine as long as they are independent and standalone. But following themes across many paragraphs is harder. It's still possible even if tired, but it's not always worth the effort (since I could just do something else now and then read it later).
When writing, it's less effort to follow themes across multiple paragraphs. Because I understand my own themes better, and I can remember my own ideas more easily – they are more suited for cheap usage by myself than other people's ideas are.
When writing, I typically focus on ideas that make sense to me and fit with my worldview. I stay mostly in my frame, my sense of life, and my world. Like I'll take someone's prompt or question, then give some of my thinking about that topic. I don't always write this way, but it's the most common.
If your mind is organized well, dealing with it should be easier than dealing with someone else's that you're less familiar with, disagree with some of the organization of, don't understand some of the organization of, and only have very limited access to. You only see little bits and pieces of other people's minds, but you can consciously interact with more of your own mind.
Why do people struggle to write? One reason is they have nothing to say – they don't have ideas. This is, largely, a lie. They have lots of ideas they suppress as not good enough, dumb, socially unacceptable, or various other reasons. Some people don't know much, but they could at least discuss what ideas they do have, and improve.
Some people are second-handed, other-people-oriented and attuned to thinking about and dealing with others. They don't see much burden there. And they are bad at dealing with themselves, alone, so they do see a burden there. These people are frequently also bad at dealing with reality.
People make mistakes, so they contain contradictions. The rest of reality doesn't. This makes people harder to deal with in a fundamental way – especially other people who come off as contradictory to you (others have a lot of contradictions that you don't share, which can be confusing to deal with).
Another issue is passivity. Reading is a more passive activity than writing. When you read, you can misunderstand everything and never notice. If you write incoherently, as part of a discussion, people will criticize – or even just ask you questions to try to make sense of it, which you would struggle to answer. It's possible to read actively with good critical thinking and error correction, but most people don't do that very well. If you're going to do something passively and fool yourself into thinking you did it well, reading works better than writing.
Big picture, if you find reading a lot easier than writing, it's a warning sign that you have a large moral flaw.