IIRC the Calendar standards (iCal, etc) are pull-based, so your Calendar app has to poll/request the whole calendar constantly to check for new events in case you make one on another device (e.g. your computer). There are also enough edge cases that also nobody wants to change/improve on the standards.

WhatsApp can send you a push when you get a message, so its usage is ~nothing if you're not sending and receiving messages.

That still doesn't explain why it has to consume huge amounts of CPU.

Hmm, it almost makes me suspicious that the calendar system is abused to move data it isn't supposed to move.