Why do they do it? I wanted to find out for once and for all. So, I trawled around for a while, read a few articles about phonetics and this is my take on it:

Remember learning about that thing in your throat that makes sounds as air passes across them, the vocal chords,  not those.. the other thing that opens and closes to makes the sound of consonants – the glottis? This is known as the glottis stop. Well, the glottis stays open  when you make a vowel sound which evidently causes a bit of a strain on the vocal chords so the ‘r’ sound is inserted to avoid this overload. Also, the pronunciation of two vowel sounds one after the other is apparently a bit of a no no in the English language because of the danger of the words not sounding as they should thereby, risking misinterpretation.

Now that I understand why they do it, I’m wondering why other English speaking nations don’t.
Are the English, as the originators of the language, the only nation to have learned how to preserve their vocal chords whilst speaking it? Or perhaps the rest of us are still subliminally rebelling against having to speak the language at all.

My suggestion is that it has evolved out of laziness, after all, it is easier to add the ‘r’ onto a word ending in a vowel. This might also explain why they drop the ‘r’ from words that end in ‘r’, say ’sikth’ instead of ’sixth’ ,  ‘ff’ in place of ‘th’ and use the unaspirated ‘h’.