many reasons:
some people have girlfriends/boyfriends
some people think they’re gross
some people dont have much money(i know you said cheap. but some people cant even afford the cheap ones. and if its every time you feel like touching yourself. that adds up really fast)
most hookers wouldnt want you touching yourself while you watch them dance
if your talking about paying people for sex thats illegal, and you could get an STD.
with porn you can just search and not go through all the trouble of finding someone
some people would be embarrassed(i would be)
some people are under aged

the list goes on but i think you get it.

u sir