Map stealing is possible because of how online games function. For you to see a game on your screen, your computer (the "client") must download information about that game's world from the server.
A newer generation of stealers doesn't just grab the map. They exploit server-to-server teleportation. The thief creates a fake teleporter that sends players to a "honeypot" server where the map data is intentionally exposed to the client before being filtered. Roblox Map Stealer