I'm 24, for reference. I grew up playing multiplayer games on the gamecube with my brother and friends from school. super smash bros melee, mario party 4-7, mario baseball/tennis/golf/soccer, wario ware inc, kirby air ride. etc. I actually remember trading my xbox classic for my uncle's gamecube because super smash bros looked so much fun.
Aside from that, there are a few points as to why dolphin is nice:
1. The roms are "free". I ripped mine a long time ago but there are a lot of rom sites online. Not that I find it unethical since Nintendo would not be making money from used game sales anyway. This is probably the biggest draw because playing a regular indie game means our entire friend group needs to pay $10-$30. I wish all my friends had copies of Gang Beasts or Tabletop Simulator, but they don't.
2. Honestly, these games are still quite fun in 2020. Nothing really captures the mood of gamecube-era mario party games. The new one for the switch is kinda fun, but remains watered-down compared to the originals. Not that I would play these daily but they're fun as a variety mix. As far as other 4-player couch co-op games go, there aren't that many that are that fun that 3 of my friends own. Gang Beasts would be fun... if anyone actually owned it.
Mario sports games are... something else as well. Aside from baseball, which has hilariously bad controls, soccer/tennis are nice mix of skill and bs. The new mario tennis for the switch is pretty fun, but I think I'm biased for the original (and all the memories I have of competitions with friends at LAN parties). Games like Lethal League are in that vein, though.
There is no other game like wario ware inc, that I've seen.
3. Computer performance hasn't been a huge issue (thus far). We've played cross-platform with linux, windows, and osx dolphin clients without a hitch. Our weakest link is a macbook pro from ~2015.
4. Pretty much everyone has a computer, whereas systems are fragmented. I don't own (and don't plan to own) an xbox or ps4, and not everyone has a switch. Or, a switch and the $60 nintendo game everyone wants to play.
I still play modern games like tf2- oh wait, that came out in 2007. Like street fighter III:3s- oh, that released in 1999, three years after I was born. Dwarf fortress? Not released... but it doesn't look like a game from 2020. Chess..? ¯\_(ツ)_/¯ Jokes aside, there are multiplayer games for the PC like stardew valley (2016), animal crossing: new horizons (2020), terraria (2011), etc, that me and my friends have enjoyed.
For pc games that have co-op, but only locally, I want to try parsecgaming [0] soon.
[0]: https://parsecgaming.com/game-streaming-technology/