Because 4chan actually severely lacks original content creators. This isn't to say drawfags mind you, but original meme people. IIRC Moot wanted to create a site of his own based largely around quick drawing for communication that died precisely because most people just don't put in the effort to make things most of the time.
We can compound this a bit with the fact that getting a meme to catch on can be hard. Wojak and pepe are embedded deeply into 4chan at this point and are used both by 4chan regulars and normies, which helps to keep them alive, alongside active actual spam from ((discord)) groups helping and forcing variants like coomer and basedjack catch on. How would (you) try to make a meme you'd want to catch on and actually have staying power?