I guess it depends why you're running a contest... but I'd broadly disagree with your belief that "Contest holder have little incentive to make them honest".

(edit: Didn't mean to go on as long as I have in this comment, ah well.)

As a marketing person I've been involved in probably about a hundred competitions for companies ranging from tiny indie game devs to major tech companies and in every case we've been highly incentivised to make them honest, we make sure the Ts&Cs are published and fair (and rigidly followed by us), that nobody related to brands involved can win, that no human element is involved in affecting the winners chosen (unless there's something like a judging element), etc.

Highly motivated because any mistake could cause both legal risks and, if any bad publicity (or a lawsuit) were to come out of it, a business contract could be at risk that's worth far more than all the prizes combined. Even data security, we comply with GDPR and do everything reasonable to keep it secure both for privacy and for competition integrity.

And that's the case despite giveaways / competitions being probably less than 1% of both workload & budgets in my work.

If I were to run a company that dealt exclusively in organising contests, I would want to be even more careful and spend even more time involving lawyers (it's already a hassle complying with different laws and regulations on how competitions can be run in different countries btw, but my current small marketing agency complies for each country allowed to enter our often-worldwide competitions), not less.

Of course that's not to say there aren't people running competitions that are intended to scam in some way, and people running competitions incompetently, both in small businesses and even in huge globally famous ones. But if the purpose of the contest is marketing (which is the case for a lot of them), there's definitely often plenty of incentive to do them to the best of the organiser's ability.

Edit: to give a relavent example from just a few months ago, some colleagues of mine organised a promotion for a client. It was a sweepstakes hosted on a website, run over about a week, which involved various ways for participants to earn points e.g. by engaging with gaming streamers as they watch. At some stage while the competition was live, somebody on Twitter called out that the wording in the copy at the top of our page was somewhat ambiguous and didn't match the exact wording in the terms & conditions attached to the promo (which were written / updated to be specific to this one promo, it just wasn't perfectly worded) - such that it sounded from the headline that if you had the most points at the end of the week you WOULD win the grand prize, whereas the terms and conditions correctly described how we intended to pick the winner, a random draw in which each point gives you one entry (one chance to win), so that if you have the most points you have the best chance of being picked, but no guarantee.

Within two hours of the tweet, during which time two other people replied (one agreeing that it was misleading, one arguing it was fine) we had discussed it, emailed the company we'd partnered with for prizes in this competition asking if we could get two of the grand prizes rather than one while deciding that if for some reason they would say no we'd just buy the second one (they said yes), we changed the wording on the website header to more clearly describe the correct terms, and then tweeted that due to the potential for confusion we would be adding a second full grand prize package (about $1k of value iirc, mostly PC gaming hardware plus a special prize from a particular game) to be given to whoever gets the most points, while still having the same grand prize to be drawn in the manner described in the Ts&Cs. The positive response from the community was worth way more than the cost of the extra prize package.