It seems like many people I work with inside and outside of the company always send an email about my availability for a meeting before sending a calendar invite. Is there some workplace social norm that frowns on sending just a calendar invite without some preliminary message? After I accept, they then send the calendar invite or they even respond by saying they're going to send a calendar invite. Apparently, they know how it works.
To me, it seems like it's easier to decline or change the calendar item. I know external users may not be certain about someone's familiarity with calendar items or how their system will handle it, but it seems to work fairly seamlessly.
I thought this may be a generational issue, but it seems common across age groups and technical expertise.
EDIT: I get the calendar isn't perfect, but I still feel (after reading the answers) it is better than just an email. Even if the whole email back and forth and reply to all is limited, you're right back where you started and have to create a calendar item. Seems redundant.