İyi yolculuklar  (can you explain if this is plural and why? )

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In Turkish ,  many  greeting, wishing fixed phrases are said in plural form , for example :

" iyi günler " ---> have good days [ have a good day ]

" iyi geceler " ---> have good nights [have a good night ]

"iyi sabahlar" ---> have good mornings [ have a good morning]

" hayırlı işler " --->  have good works [ have a good work]

 " tebrikler " --> congratulations

" iyi yolculuklar" ---> have good journeys [have a good journey]

Turks are geneorus people...Why keep a tight fist and use singular form when handing out wishes?

They are all free, aren´t they?