I find the exact same thing. I always use an alias when signing up for some stupid parking app or new healthcare system that I can't avoid and I've surprisingly never seen the aliases being re-used by anybody but them.
I was expecting at least a data breach, but it's been clean for the past 10 yrs or so.
It's possible data brokers are just good at stripping whatever is behind the "+" though.
Forums and websites have always used content to market things (think DAU or visitors to sell more expensive banner ads at the top of a post).
I agree that forum hosts were (mostly) random people that wanted to do something for their community, but monetization didn't kill the friendly Internet, consolidation did.