Antivirus / Anticheat detection

I was wondering how people keep making their hacks and viruses/malware/keyloggers/other undetected. Isn't there like only a few read memory functions in C? How come the antivirus/anticheat programs cant just scan for anything tapping into its memory through the function? Obviously im new to hacking, but i know some basics of programming (Mostly Java, but that isnt the best for hacking XD). Thanks!