Why do a lot of English people drop the r at the end of words, but add them where they don't belong?

I don't harf unduhstand it.

This is a matter of regional accents. You won't hear educated Londoners doing it for example, who most closely fit the stereotype of "English language masters."

I keep wondering that myself. Drives me nuts the way they say idearr

fuh'in' bangin' m8 sau'ed

It's explained in this Wikipedia article: https://en.wikipedia.org/wiki/Linking_and_intrusive_R

Because English accents became predominantly non-rhotic in the mid-nineteenth century. You'll still hear rhotic ones regionally, for example in the south-west of the country.

Australians, Kiwis, the Welsh and South African English-speakers are mostly non-rhotic too. So it's not a tendency that's confined to these isles.

As for why the English would imagine themselves to be the masters of their own language? Who knows? It's a mystery that will probably never be resolved.

Like Americar?

I think you mean people from Boston. The Cah in the Hahvahd Yahd!

I do notice that some upper crusty Brits use a “w” in place of an “r”.