To be fair, most of the extra features (including this one) are apps that you either have to enable or can disable.
And even though I initially installed Nextcloud as basically an FTP alternative too, I absolutely love how I've been able to migrate my contacts and calendar over (away from Google) with basically no hassle.
It is all apps. You can even disable sharing. Heck, you can disable the Files UI...
Note that most people use Nextcloud to work with others and features like these are crucial to their productivity. In the end, we're guided by community contributions and customer demand... Luckily you can disable things you don't like :D