Confusion about variable capacitors/diodes
Do I miss/oversee something here?


Varicap diodes vary the capacitance by varying the DC voltage on reverse biased PN junction. PN junctions don't have a massive amount of capacitance hence you only get several tens of pF for something like a BB171 made by NXP: -



The data sheet refers to it as a VHF device but I use it at frequencies from 1 MHz to 15 MHz.


  For resistors there exist variable resistors ICs, and I would expect
  something similar for variable capacitor ICs


There is one digitally controlled capacitor that i have come across but, like the varicap diode, this was for applications that required a capacitance in the tens of pF range. I guess there is no massive reason why bigger value digitally controlled capacitance chips couldn't be available other than market demands. 

I mean, you can make a low-speed to medium-speed oscillator by varying the resistance and if you wanted a high-speed LC oscillator most varicap diodes are in the right capacitance range to suit the higher frequencies.