The QT3 was designed to provide a bubble of clean air around the breather on the go. As such, the gaps between the filter and the QT3 HEPA filter were intentionally designed so as to increase the airflow from the QT3 air purifier, to increase the bubble size.
If the HEPA filter was sealed to the frame of the QT3, then the airflow would be drastically reduced. That means less overall airflow, within a smaller range from the QT3. For those using the QT3 as a cooling fan, it would also drastically reduce the QT3’s ability to provide cooling air.
Dyson’s air purifiers use essentially the same ‘technology’ in their purifier fans, calling it the ‘air multiplier technology‘. They market it as providing an increased airflow that can clean a bigger space, similar to what is explained above about increasing the size of the QT3’s clean air ‘bubble’.