Operation of a MOSFET
The transistor, mos and bjt, is a highly non-linear electronic device. A resistor is mostly linear. Apply a voltage across it and you get a current. Increase the voltage by a delta and you get an increase in current proportional to the delta voltage.
For your mos simulation, remove resistor R1 and start with a small VDD, say 1 volt and find IDD. Increase VDD and find IDD again. Repeat to form a table, IDD for each VDD and sketch plot IDD as a function of VDD. You will see a rising IDD at first followed by a slowing of the rise rate and finally a region of 'saturation' where the current rises only slightly as VDD increases. This is the characteristic MOS curve.

Now repeat with a different VGS, say 8 volts. You will get a similar shaped curve but saturating at a larger current level. This shows the gate control of IDS. Try that and post. Rds is the reciprocal of the instantaneous slope along these curves and is a function of Vgs and Vds. 

So, Rds depends on the operating point of the transistor. Is it in triode or saturation or in between? For your case, Vds is high so that the device is likely to be in saturation where Rds is relatively high. The device operates more like a current source with a large shunt resistance (Rds) so that the output voltage becomes Vdd-Ids*R1. Ids is limited by Vgs. As you increase Vgs you will get a larger Ids and the Vds will drop. Making Vgs very large will drive the device into triode, Rds gets 'small' and Vds will further decrease.