> To subscribe to a new calendar, you must use a computer web browser. You can't subscribe to a calendar in the Google Calendar app for Android, iPhone, or iPad.

Isn't this likely because Google Calendar was abused a lot in phishing? e.g. fake "CLICK TO PROCEED" links as ads -> redirected to Google Calendar named "CLICK [+] SIGN TO CONTINUE" -> "URGENT: re-enter your credit card number at https://scam.example/buy-us-bitcoin" fake schedules recurring every x minutes etc.