The reason you're not aware of them is probably—assuming you're a guy—because they're mostly played by women (especially the ones that aren't outright porn—though, those too). They're particularly popular on mobile. If you've ever seen the stats about how there's a huge number of "women gamers" and been like "wait, if there are so many, where the fuck are they?" it's because a large proportion of them mostly play dating sims and visual novels (often those two have more than a little overlap) and other game genres that you don't, like casual browser games or puzzlers.
It's similar to why the average woman can name more romance novel authors than the average guy can.
In Japan, dating sims used to be huge, Tokimeki Memorial being notably famous. Unfortunatel, there seems to have been a quite aggressive decline.
When thinking of currently running dating sim series, I can recall Tokimeki Memorial: Girl's Side, ×CATION, and a recent trend of remakes (such as Doukyuusei). However the fervour the genre once had is noticeably absent from current releases.
With rare exceptions (like Hatoful Boyfriend) these joke dating sims feel pretty fake. They comment on a genre they struggle to properly portray or understand. The most egregious cases parody galge or otomege and erroneously label themselves as dating sims.
As an example, Doki Doki Literature Club (for all its merits), suffers from being described as a dating sim when it clearly isn't one.
To be clear, while pure daring sims kind of barely exist, they do pop up from time to time as secondary gameplay loops in hybrids. The Persona series (3 and onwards) as well as the Fire Emblem series (Awakening and onwards) do this.
I believe Steam has had a massive uptick in them over the past few years.
Matchmaking your soldiers together to find romance on the battlefield is a common trope and basically is a dating-sim with regards to "guess the character's likes/dislikes". (Giving gifts to various characters to earn romance points in Fire Emblem: Three Houses, for example)
There's just, also a whole battlefield / tactics game going on at the same time. But the dating-sim portion gives extreme benefits (support-class A or S gives huge bonuses that allows more attack/defense/dodge).
They're usually a form of interactive novel almost, but some are more advanced.
Even Stardust Valley has a "dating sim" portion.
By the developers of Genshin Impact.
There was a fun thread where someone played through it on, er, SomethingAwful, possibly? Some old forum.
(Strictly speaking I think it was a bit of a Jane Austen mashup, but mostly Pride and Prejudice; if you screwed up you could end up marrying the awful vicar, for instance)