Maybe you only downloaded the update and not the installer?
I'd recommend you reinstall the game https://www.phantasystaronline.net/download/Ultima-PSOBB.zip
Keep in mind this file is protected by a password, the password is ultima
When reinstalling, I'd recommend to turn off your antivirus and before turning it back on, add the game folder to the exclusions list.
As for the game downloading the launcher every time, it probably is because the launcher is being removed by your AV.