Because, as an instructor, you are certifying them as competent to carry out a given level of diving.
Does it not seem pretty hypocritical to hand out plastic cards certifying someone at a given level, then desperately plead with them to NOT to conduct dives at that level?
Isn't that basically what we do with every OW student? I mean, you're being extra dramatic there. But, the bottom line is that we don't hand new OW divers their C card and then tell them "sure, you are now totally clear to go out with your classmate here and go diving completely on your own for whatever shore dives you come across. Or get your other buddy to take you out on his boat and just drop you in the drink."
We tell new OW divers to be cautious, use good judgment and common sense to gradually work their way up to diving at the limits of their certification.