Can someone explain? Honestly, because I have never fathomed: Why has calendaring and associated PIM-stuff been grafted onto email specifically? Is it a spill-over from the strange land of Outlook/Exchange? My Thunderbird does really decent service handling my mail. If I need to look at a calendar, I'd typically open a, you know ... calendar application.
It's a spillover from the strange (but comprehensive) land of enterprise communication and productivity software like of Lotus Notes, Exchange, and GroupWise, but also influenced by the web browser "suites" of the late 1990s.
Email naturally gives rise to contact management; in corporate settings most of this is derived from org-driven data, which can be laid out in a hierarchical directory. These often support LDAP.
For management of personal contacts, this is imperfect. In the mid-1990s, PDAs and web pages made a detached 'contact' datatype useful, so vCard was developed by the Versit Consortium [1], the makers of vCalendar. These efforts were later transferred to the Internet Mail Consortium, where vCalendar was renamed iCalendar, but vCard kept its name.
Right around this time IETF's RFC 2425 [2] pondered ways to express and map vCard-like contact information in other protocols and applications, like Email and Directories. And work on WebDAV was spun up to bring more structure to distributed authoring with HTTP.
WebDAV was hardly a sweeping success, but it provided a reasonable alternative to proprietary protocols like those of Microsoft, so further work around bringing calendaring and contact management on top of WebDAV continued in the early 2000s. This brought about CalDAV, CardDAV, and GroupDAV, and enabled more of these applications to interoperate.
This Osmo [1]? Well, good for you. But this software is not capacitive/finger friendly. Its for resistive/pen usage. The rest of us need a mobile e-mail and PIM client which works with a capacitive touch screen (and fingers). Also, many do want online synchronization so that their desktop, laptop, and mobile phone are synced. You can even self-host with Nextcloud as an individual or company.
Lots of good answers already, but one fundamental answer is simply that the first vaguely interoperable spec for events with attendees offered email as a transport mechanism (iMIP). Everything since has fallen back to that.
