That's the way it works. Valheim is NOT a MMO. Stop thinking of it like one. It's meant for small groups for co-op or individual players for solo-ing.

The game has framework that allows people create dedicated servers so they can have a world where their friends can log in at any time rather than waiting for a specific person to be online or because they want a more populated world, or both. It costs THEM time and money to set that up. As such, it's their rules. Don't like it? Too bad.

Again, the game is not a MMO.

As has been suggested, create a server with the password in the title and you'll quickly understand why most lock their private servers down.

Go play Rust, Ark, World of Warcraft, Guild Wars2 or Sea of Thieves if you want the "I want to play against random people" experience.