You're blind-trusting someone to run stuff in the context of your terminal. Sure, it's similar to an installer but the author of the script can also manipulate the script at any time.

One day you run it, it's fine. The next day you run the same command on your machine, it installs malware. No way to tell without inspecting the script every time.

If you download an installer and it's fine, then you can run it again and it's still fine.