They do. The problem being, marketing chans to other chans is a waste of time. The days when you could announce the launch a new site with a spam raid across 4chan are long gone. Users who remain are hold outs from simpler times. At best, some people will stop by, notice nothing is going on, and skip back to whence they came, never to return.
Potentially useful newer users are already extensively active in Discord, websites that directly focus on their interests (DeviantArt, many boorus), wikis, and the more traditional forums. Like old school TV, chans aren't places for the need-to-be-seen advertisers any more.
Launching a chan these days requires:
a) A preexisting user base willing to give it a go. That, more or less, made 4chan's start.
b) Or, a topic or movement not already covered by another website, or chan board. Redundancy is deadly.
c) Failing the above, you need to stomp on the coattails of an existing group being kicked from somewhere else, and actively guide them to your site. Good luck timing such migrations. You have to be in there right from the start, and move faster than everyone else.
What doesn't work:
a) FEATURE RICH! THE NEXT BESTEST THING EVAH TO SEE THE LIGHT OF DAY IN CHAN TECHNOLOGY! There are dozens of different image-board systems with features you've never even heard of. Nobody cares, unless you bloat up and sink so far down the hole you're not running a chan anymore. You really want to compete with Discord?
b) TOTAL PRIVACY AND ANONYMITY FOREVER AND EVERYTHING! The people who insist on doing things this way moved elsewhere long ago. Sites offering you such while connected through the traditional internet are lying, even if only to themselves.
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