Hello
To answer your question simply "Why do people play on the FiveM server" - Because it's fun.
But to expand on that people play on the FiveM server because it adds a completely different experience compared to the Arma 3 Altis Life. In Altis you have militarised gangs and rebel factions with massive power and influence on a certain area of the map, along with a para-military type police to combat everything. This creates a "Warzone" type enviroment where it's a "Dog eats dog" world.
On FiveM it's a lot more relaxed, sure there's still crime. But instead of massive militarised rebels and gangs roaming around you have petty street criminals, maybe someone with just a knife who wants your few £ left in your pocket. Or maybe you run into an Organised gang, or Mafia, or maybe you run into a cult? The possibilities and variability in FiveM is endless compared to Altis.
Altis follows a very specific formula and doesn't allow for as many unique RP scenarios, for example, being stopped for passing a red traffic light. It sounds simple and basic, but it adds depth.
The FiveM server is rough around the edges and needs to be filed down, which i am sure the Dev team are working super hard on.
What i will say is that FiveM is free, and GTA V was free (It's 50% off now on Epic), so try it out. Takes only a few minutes to set everything up and you can try it from yourself and get a feel for everything. As i said, it's rough and unpolished, but it's off to a good start. Expect different types of RP. This isn't Arma, the combat system is very basic (Point and click) so don't expect Arma-esk combat.