Hi @Ân_Vũ_Trọng and welcome to the forum,
I agree this would be a great feature and just realized I hadn’t voted for it yet, so I just added my vote.
As to your puzzlement on why it hasn’t been implemented yet, it’s a matter of overall prioritization, not just degree of simplicity. Asana has literally thousands of things that people would like to see changed - some reflected here in the forum, some not - so it’s a huge juggling act to choose which ones to do when.
For a more complete answer, see this post that a few of us Forum Leaders wrote to try and answer questions like this: 🦄 If Wishes Were Unicorns: Why Asana Might Not Have Implemented that Feature Request (and other thoughts about Asana feature development and product strategy)