Why they pronounce idea like "idier"? Sometimes they do the same thing with some other words too.
It is simply a fact of English pronunciation in certain parts of the world.
In southern England, for instance, there is a regional habit of pronouncing words that end in 'a' to sound like 'er'. So:
Pasta/Pastor sound the same
Outside of southern England, I do not think it is very common. As someone who grew up in the southern UK, I know that North Americans find it both confusing and humorous.