Because you are not changing focus as often as usual and also people tend to stare and not blink as often as they normally do. That's why regular breaks are recommended.
its the same as any other activity where your eyes are fixed at one focus distance for any length of time. Thats why you should blink, look away and refocus every so often.
Because of the 5 senses you have you are predominantly using just 1. The answer is to put on the radio so that your audio senses kick in. The brain does not act in parallel but short sequences are happening so the demand is not confined to one task. This is all happening in the millisecond phases so it seems you are doing more than 1 thing at the same time.
An obvious example of this is if you used a mobile phone while driving. Most people can either do one or the other but not both at the same time
This sometimes happens with younger people with sharp minds. Without realising it, your brain is conscious of the refreshing of the screen image. Take breaks as recommended above.
A couple of decades ago this was a frequent complaint with cellulose films at the movies as their brain was able to detect the flickering from one image to the next. The same reason why different people detect the start of the perceived 'reverse motion' of the spoked wheel in films at different times