So many areas of needed fixes and improvement, and they're working on emojis.... #srsly
Are you also confused, that Apple has people working on any other software products, marketing, hardware design and customer support, and people doing gardening and catering, all the while there are bugs in OS X that need to get fixed?
Most roles are highly specialized, and not interchangable, not even within the same product team. So someone working on the emojis could not just quickly be thrown to work on memory management and file systems, without familiarizing them to those parts of the system. It would take time and resources to train them, and that would not help getting bug fixes out any quicker.
Having someone fix a bug in the part of the system they are specialized in, is not going to slow down the improvement in other areas.