That does not match my own experience.

Back in my day, I received a couple of demo disks packed in with my PS1, and I got a couple more through other means like magazines and the famous Pizza Hut promotion. Some games would include demos for other games too: Spyro and Crash Bandicoot used to do that a lot. Now that I think if it I don't remember ever seeing any cartridge-based demos at all for any Nintendo or Sega systems, even the later ones like GBA and N64. There were kiosks in public places, but I never saw anything intended for a consumer to have- carts were just too expensive.

By the PS2 era demos had mostly dried up. I have a God of War demo that came with a magazine and that's about it. I could only speculate as to why, but I suspect increasing game sizes and DVD's being more expensive than CD's may have been a factor?

I'll admit I stopped paying attention to demo's for a while, so maybe I missed a peak at some point from like 2010-2020. But nowadays Steam, the Switch, and the PS5 all have a category or filter option to look through demos. There's tons of indie games trying to get attention, and of course tons of shovelware too. But most of Nintendo's published games have demos on Switch. Scrolling through the PlayStation store I see EA sports games, Persona 3, Power wash Simulator, Ys, Diablo 4, FF VII Rebirth, Tekken 8, Crow Country., Chants of Senaar, Sea of Stars, Ghost Trick (I didn't even know that was on the PS5 lol), Like a Dragon, Resident Evil 4... So Square, Capcom, Sega, EA, Activision-Blizzard, tons of indies, and more. Sony is the only publisher whose absence I noticed, unless you count VR stuff. The number of demos available today is overwhelming, if you look for them.