It's a function of how much stimulus a task involves, how much disambiguation you have to do during the task, and how much context you need to load to perform the task (and how different it is from previous tasks).
e.g. video meetings are more stimulating than audio meetings because you have more sources of input, and you have to stay alert in real-time. A single email is less stimulating because you can process those in your own time and space out if needed. But a morning full of answering emails is worse than a single video meeting because each email involves loading new context into your brain.
A sibling comment addressed disambiguation already. It's just a lot of work to take input, process it, be aware of context and goals, and get to sensible output. These days, I tend to allocate time just for planning. Executing plans is easy and I can do that when I'm tired, but planning takes a lot out of me.
I'm sure the cognitive impact of exact tasks is different for everyone depending on what your brain is used to. e.g. I'm very introverted; maybe meetings are less work for others.