This is done on purpose. Per-project library management should be preferred in any case imaginable. See PlatformIO IDE 2.0 for VSCode 🚀 - #12 by ivankravets and Unable to install library to global storage via GUI.
Global libraries can still be installed by opening a PIO CLI and executing a pio lib -g install <library name> command. (example: pio lib -g install itead/Nextion@0.7.0)