Why wouldnt DJs use streaming for their gigs?

please don't eat me- I have only just started learning to DJ!! Will probably never become a real DJ 😀 but I'm having lots of fun With streaming services available and being able to connect them directly to tools like Traktor (this is what I have), why do DJs still purchase tracks that are so expensive? Is it for the security in case if the venue where they play doesnt have wifi or suddenly there is a wifi problem?