Dielectric constant (relative permittivity)
Because the concept is easier to teach &amp; grasp, and the equations easier to work with, if it's assumed "constant".  

When learning about diodes, we simplify them - say they're "non-conducting below 0.7V" and "conducting above 0.7V", but that's not really true is it?  They have a highly non-linear range of conductivity.   But taking that into account makes circuit analysis ALOT harder, so usually we ignore that fact.   

One of the secrets to being a successful engineer IMO is knowing when you have to consider the secondary effects, and when using a simpler model suffices. 

If considering the wavelength dependency of the dielectric 'constant' only changes ones answers by, say, 0.1% (just throwing out a number here, don't read too much into it...)  and we have a circuit full of 10% tolerance capacitors, does that error really matter???

On the other hand, if Einstiens relativity weren't accounted for, GPS accuracy would noticeably suffer.  Very small effect there (time dilation... time moves a hair faster for satellites than it does for us earthbound folks), but over the large distances involved, it becomes a noticeable error.  

The Scientist says "This value is 1.09482 +- 0.00002"

The mathematician says "This value is 1.094829384929375092835012084376917349509345..... to infinity"

The engineer says "Eh... 1.1 is close enough"