Because:
There’s no way to sync contacts and calendars between an iPhone (and other mail clients) and protonmail. The app does one way sync from the phone to protonmail, but not the other way round.
8 years ago a feature request was made to add support for CardDAV and CalDAV, but even with the release of bridge it’s not there.
So iOS users have to resort to using other calendar services, or 3rd party bridges to enable it.
Yep, This is taken straight from Facebooks advertisements circa 2018, maybe still today.