My take is marketing is a lot of work, a lot more work than many people want to do.
When I was in college I ran for election to be the PR director of the college radio station because I wanted my own set of keys to the station. It was my job to promote dances, but KTEK dances had an awful reputation of having no attendance.
For the first few dances, I resolved to do a massive promotion campaign that involved printing several hundred 8.5x11 poster for a campus that had about 2000 students. That brought a big crowd and fortunately my mates had my back because they did a great job of DJing and it was a big hit.
After a year of this our dances had a great reputation. We slacked off on the promotion a little but not much.
I'd be loathe to delegate this kind of task: if I told many people to print 400 posters many of them would print 4 or 40 and already think they were working too hard. It just takes a lot of work to get results, possible 10x or 100x what you're thinking.
I've worked for startups that were well funded and well-connected could "pull strings" to get mentioned in TechCrunch and frequently they'd get some special opportunity and fail to capitalize on it.
There are thousands of places where you can promote online. Although paid marketing can be effective it is worth posting links to sites like HN, forums, emailing bloggers, etc.
The other half of this is generating content which has broad interest. If content is interesting to you because "the cash register rings when people click on it" then it probably won't be interesting to others. Just as many people need to overcome laziness to promote effectively, many people need to escape their own self-centerness and get some insight into what other people find interesting.
One success used a little guerilla marketing. We made a fake review site with only one article and shared it to a targnt market Facebook. 3000 downloads in a day, with too many orders to handle myself. I built the shopping "cart" to handle maybe 5 orders a day, but IIRC we got around 90. People were excited to have an app for it and shared to their buddies doing the same thing.
Another one ranked well on search engines, but it took years. I did mention it on some places and sites: Reddit, HN, a forum I visit. It got a traffic increase of 1100% from 2020 to 2021 from 8/day to about 90/day, but only a 9% increase from 2021 to 2022 after I neglected the site. It's not exactly viral, but there was almost no marketing effort. If people like it, they share it. It's being covered by at least an instagrammer and a YouTuber, no major ones, but word catches on.