Because in this sentence mucho isn’t “many Cuban restaurants”
It is “really like Cuban restaurants”
If you wanted to say “I like many/a lot of Cuban restaurants you’d say “me gustan muchos restaurantes cubanos”
But we are saying “me gustan mucho restaurantes cubanos” Meaning I really like Cuban restaurants.”
Mucho belongs to how much you like something versus muchos belonging to how many restaurants you like.
I hope this helps
This translation came twice in this topic. First time I answered as per your suggestion here, but you said that it was wrong, and suggested this: I am very much into Cuban restaurants. That is absolutely silly! Now that it came around the second time, I used your translation that you suggested to be right. Instead, you are telling me that my first translation was correct: I really like Cuban restaurants. Can you make up your mind what is correct??!
Hello ShreshthaS. Simply because that's not how it works in Spanish. With Mucho in this specific sentence you modify how much you like something at the end of the sentence.
Me gustas mucho. Me gusta esta lampara mucho. Me gusta comer pasta mucho.
There isn't really a steadfast "grammar rule" it's just how it works in Spanish.