Because the game doesn't open without the server letting it? They needlessly built it so to thwart piracy. They explicitly state that offline play is not allowed nor is it planned to be allowed, and that a constant internet connection is required:

"We don't have an offline mode currently planned. Temtem is built to be 100% online and making it offline is not a trivial task"

"An internet connection is required"

This is a small indie company that will eventually pull the servers, and that's only if you don't get banned before then:

"Friendly reminder that abussing exploits, using bots or macros or using any kind of cheats/hacks can get you banned from the game."

Every single Pokemon game is playable forever (most with no possibility of ever being banned or having your license revoked) on their original devices, multiple official ports/rereleases, and emulated on PC