My initial thought was that in certain English accents, including some from the northern parts of the UK, the r is "flapped" or "tapped" similar to an r in Spanish. With a tapped r, the pronunciation of "Rick" is much closer to that of "Dick".

https://en.wikipedia.org/wiki/Pronunciation_of_English_/r/#V...

However, further research has turned up that rhyming nicknames have been a thing going back centuries and "Dick" is one among many.

https://www.thrillist.com/news/nation/how-did-dick-become-sh...

https://www.youtube.com/watch?v=BH1NAwwKtcg

https://en.wikipedia.org/wiki/Dick_(nickname)