Also, to be as effective as Google Calendar, you'd have to end up integrating it with as many external sources as Google Calendar, decreasing any privacy you might have gained by making your own product.
There are already various servers you can set up at home or on a personal server based on CardDAV, which is an open standard for exactly this purpose. I set one up for myself in a day, and I can easily access it on my phone and, from a web UI, on my computers, in my email client, etc.
It's fairly widely supported and you can use it with various calendar apps and programs, but unfortunately the standard is a bit awkward and it's still not as widely supported as things like Google Calendar.
1) Because it would take months.
2) Because it would probably be hard to make it integrate with other things like Calendly.