Well, simply, when a crime becomes widespread, we do seem to know that it exists. You're going to know about, or hear about, instances of the crime, even if you can't get prosecutions, and you're going to get at least some prosecutions started, if not convicted. I'm sure that the law finds only 1% of the pot smokers out there, but we are not unaware as a society that lots of pot smoking is going on. Vote buying requires a secret conspiracy of hundreds, even thousands to be effective, and reports of it, even rumours, are extremely low. (On the other hand we have lots of rumours of all sorts of election manipulation, some of which may be going on but much of which probably isn't going on at all.)

It's the sort of thing you would hear rumours about, even if you couldn't nail them down. People would be claiming, even falsely, that they got offered money for a ballot.

Vote buying usually requires you find people who don't care that much about their vote, are very poor and of course would not have voted for you otherwise. Not the world's best secret conspirators.