It's challenging to get networking right, and the effort required doesn't get all that much smaller just because your game is smaller.
Most engines do come with a networking framework or layer these days but Roblox gets to assume a bunch of things an engine can't, and as such provide a complete solution out of the box.
Everything was replicated in the client and server. So you could open Cheat Engine, modify your total $$$ on the client, and it would propagate to the server and everyone else playing.
They only fixed this in 2014 with FilteringEnabled/RemoteFunctions but that was opt-in until 2018 and fully rolled out in 2021 (breaking most classic Roblox games). This also made games much harder to develop.