The others are focusing on some things that it could be including demotivation, burnout, disinterest in learning something that is just a different version of what you already know, etc. Those are all definite possibilities.
However, another thing that could be happening is that as you become more senior, you start being used to knowing things really deeply, and so when you learn something new two things happen. First, you are comparing your knowledge of the new thing to your knowledge of the old thing. When starting out, you don't know anything, so when you learn a bit of React, for instance, it seems like you are learning a lot. But if you know React deeply and learn a bit of Vue, it feels like you have a long way to go.
Secondly, you want to know things deeply, and so you question and analyze all the knowledge that is going into your brain. When starting out, you are happy to just repeat knowledge without understanding it deeply. An example I use is when I started at my current company, there was an intern that started at the same time. We both would be told, "here is a script to put things into the staging environment". She would take it and use it and move on with life. I would question what it exactly did, why it did it that way, what were the failure modes, and the history of how it go there rather than other options. She was much more productive to start, but fast forward 18 months, and I have a much deeper understanding about our technology and codebase and approaches, and am able to drive long term decisions and fix deeper issues.
I can echo this, and am of a similar age as the OP. Even things I feel excited about learning are chores now, simply because my expectations are grok it quickly and be productive. The act of being bad at it, not understanding something, having to look things up, not having the full shape of everything in my head and so sometimes having to cargo cult it until some later date when it'll make sense, etc, are alien and exhausting now. It's hard to pour myself into something I'm "bad" at, when there are other things I know I'm "good" at.
However, another thing that could be happening is that as you become more senior, you start being used to knowing things really deeply, and so when you learn something new two things happen. First, you are comparing your knowledge of the new thing to your knowledge of the old thing. When starting out, you don't know anything, so when you learn a bit of React, for instance, it seems like you are learning a lot. But if you know React deeply and learn a bit of Vue, it feels like you have a long way to go.
Secondly, you want to know things deeply, and so you question and analyze all the knowledge that is going into your brain. When starting out, you are happy to just repeat knowledge without understanding it deeply. An example I use is when I started at my current company, there was an intern that started at the same time. We both would be told, "here is a script to put things into the staging environment". She would take it and use it and move on with life. I would question what it exactly did, why it did it that way, what were the failure modes, and the history of how it go there rather than other options. She was much more productive to start, but fast forward 18 months, and I have a much deeper understanding about our technology and codebase and approaches, and am able to drive long term decisions and fix deeper issues.
Learning as a senior is exhausting :)