Nige321
Because a UAV of that size isn't really available off the shelf. It takes some knowledge and skill to assemble, set-up and fly.
It simply isn't the kind of vehicle flown by people like this.
I refer you to the post above that... 3 different pilots, 3 different aircraft from 2 airlines. Reported independently in quick succession. The first reckoned they missed it by about 30'. The other two saw it after the ATCO warned them of the initial encounter. All three agreed on the description of size and lights...