I actually thought this as well. When I created a TOC for a 700+ page book recently. It took forever lol. And I kept thinking if there was no drop down box it would save me a couple of seconds each chapter.
The delay after clicking the header selection for me is like 15 seconds before I can then scroll down to add another chapter. One of the problems is that if you accidentally hit the wrong header (which happens sometimes using the drop down box) you have to wait those 15 seconds and another 15 seconds for the correction. For me it takes 1 minute in down time to create 4 chapters lol.