My sims seem to have have selective attention spans, as in when they need to go to work, that is when they'll conveniently keep using the computer, or playing the guitar etc, then I realise they are missing work.
Mostly my sims don't seem to have a short attention span, though. The only thing that really annoys me now is that there is a nooboo in my household, and when he cries, I click for my sim to feed it or whatever, but then she doesn't because every other sim has stopped what they are doing to go and attend to the baby! My sim that was trying to do the "maintain focus for two hours on video game" task kept getting up to attend to this baby which isn't even hers.
I also found my sims will stay on task, like cooking a meal, but then someone always has to come and chat to them and it ends up taking them two hours to make that salad because they stop to talk.