Why return loss gets ripples?
The simulated return loss S22 is smooth, but S22 of the fabricated die shows ripples


What that means is that what you are measuring is not what you simulated, period. The difference between them can be interpretted to hint to you which part of the model is wrong.

What you simulated is probably a perfect network analyser, connected by perfect 50ohm line through perfect connectors, to a circuit which is modelled exactly correctly.

A common problem is a length of line that's the wrong impedance, or line between two connectors each of which has a reflection, connecting the ana to the device. This will set up an error which is periodic in frequency, with a period related to half the line length. 

If you calibrate the measurement system to a reference plane within your test piece, this will mostly take out the influence of the ana, the connecting line, and up to connector repeatability, the connectors. 

The difference in modelled and actual traces suggests a ripple that does not look periodic, suggesting to me that it's not such a line resonance. It does however look suggestive of the increasing frequency of ripple that one gets as one approaches the corner frequency of a high order filter. Perhaps you have some big unmodelled L and C equivalents somewhere, for instance where the transmission line changes profile. 

If you have suitable software on your ana, you might try to simulate a TDR view of the difference. This will show you a time domain interpretation of the impedance errors which might hint to where the major deviation is.