By the same logic, why does he leave it up to chance who will find them in the first place, given that he was clearly looking for a specific type of child to visit him? After all, the primary purpose of this was to select a successor - selecting 5 kids at full random and choosing his favorite seems like a very odd way to choose a successor (especially given that 4 out of the 5 - 80% of the candidates - were clearly completely unsuitable).
It's unknown the extent to which Wonka was leaning on the process (if he was at all). The fact that Violet's father had his entire factory looking for the Wonka bar would suggest that that one at least wasn't the result of him leaning on the process, but it's quite plausible that Charlie's was no accident. (After all, the previous 4 did seem to be unsuitable for some rather obvious reasons even just based on the news coverage of them, so it would seem rather absurd for him not to make any effort to lean on the results of the final one). Obviously, the text doesn't directly answer that, though.
That being said, the rules of the contest do specify that only children were permitted to win the contest, so presumably if an adult had found the ticket they would've been obliged to give it to a child.
Given the previous facts, though, another possible answer is that "he assumed that children would win for the same reason that he believed that at least one of the winners would be suitable to be a successor." This suggests that either the process wasn't as random as it looked, or that the author simply intended for us to suspend our disbelief on the point.
It is, of course, possible that it really was sheer good luck on Wonka's part, or that he believed that 5 children was enough to give him good odds of finding at least one suitable successor; by sheer statistical odds, that would require that at least 20% of the world's children (1/5) were suitable as successors (i.e. had the characteristics he was looking for). Maybe he was optimistic that a kid who hadn't been "spoiled" yet could be trained to be his next successor.
One final possibility (and this is obviously speculative): it's quite possible that there was a "Plan B" to find a successor if this plan didn't work out. If Charlie had ended up not working out, then maybe he would've just run another contest or something like that to get more candidates.