I've made money with parked domains before. (Not a lot, but some money).
Here is how it worked for me.
Years ago I used to buy a lot of domains to turn into niche websites. Sometimes those domains were already getting traffic when I bought them. In a number of those instances I would throw up a one page website that targeted a keyword phrase that the homepage of that site should naturally rank well for. That means targeting something that had the domain words as keywords. It would usually be a one page site with excellent on-page SEO. I would 301 redirect everything to the homepage. So if traffic was coming in from a link to hxxp://mysite.com/some-other-page.html I would redirect that link to point to the homepage.
I would throw Adsense on that homepage putting large blocks of text ads right in the highest heatmap area of the page.
When people tried to come to any page on that website they would land on the homepage. I assume that most of them would realize the site is basically dead. A lot of them would click one of the ads to leave the site. And that is how they made money.
I think the most I made from one site in one month doing that was about $500 (that was one site for one month). And I think the site that earned that cost me something like $50 to purchase the domain. Obviously it was a good investment.
That site didn't earn that much every month. Most months it would earn like $30 or something like that. It only jumped up to over $100 per month when the site got a burst of traffic because the topic it centered on was popular in the news for a while.
Parked sites would only earn money if the domain was already getting traffic when you bought it. So, you wouldn't register a new domain and park it and earn money. You couldn't do that because the site wasn't already getting traffic. It only worked if the domain was already getting traffic.
Therefore, the vast majority (probably 95%) of parked sites would not make any money at all and certainly not enough to pay for themselves.
Why not just study what worked with the right sites and replicate it 100 times?
I suppose you could. Here is the thing though. When you buy a domain like that it is because the site has been abandoned. If the site has been abandoned, then the traffic is already on the decline. As the traffic keeps declining the site will earn less and less. So, it can be profitable if you kept identifying new domains and you kept parking them like that every month. Then you dump them when the traffic dries up to where it is no longer profitable. I don't like the business model though. I like investing in things that are going to increase in value over time, not decrease. That is why I don't like the idea of making domain parking a regular business.
In my experience, the domains that made the most money as parked domains were websites that used to be for legit companies and ideally ecommerce websites. For example, if you went and bought a domain for a store that had been moderately successful at one point in time, but ended up closing, then that site is probably getting some buying traffic still. That buying traffic is the type of person who will click on Adsense ads. So, if you find an old company website that still gets enough of this traffic because it still has lots of links from old forums for example, then it can be worthwhile. I think you will spend a lot of time looking for a good candidate though. And you won't know for sure if it was worthwhile until after you do all the work. That is why a lot of people won't bother with it.