[JasperM]

1) I play SA:MP mainly because I started doing so a long, long time ago because my PC was too bad for GTA 4 and 5. As soon as I got a new PC, I bought GTA 5, but although it was fun in Single Player, the Online gameplay was kinda annoying for me, since higher ranked players have a real advantage and if your friends don't play it aswell, you're kinda lonely. In GTA:SA, as you mentioned, there are mods for private servers, which makes it a whole lot more fun. SA:MP is kind-of the more-attached-to-the-original-game mod where-as MTA has more possibilities and therefore doesn't feel like the original GTA:SA anymore (custom GUI's, automatically downloaded skin mods).
Besides, the GTA:SA online community is a lot more friendly and less toxic than GTA:Online where people randomly hunt you down for no reason.

2) Not an Administrator nor a scripter at the moment.

3) SA:MP is so close to the original GTA:SA game, and with the private servers it doesn't get repetitive, which GTA:Online does get after some time.