Fair enough. I do say "carry-okie". I got used to saying it that way before I learned the actual pronunciation. Likewise, when a Japanese speaker says "Kurisutofā", not only are they altering the actual sounds (changing the end from "er" to "ah") but they are introducing another syllable.
Maybe the problem is more translation and not actual speaking ability. Did those Japanese students ever learn to pronounce "McDonalds" correctly ("Mac-don-ulds") or was it simply beyond their ability to say those phonemes?
Other way around. These were Americans being taught how to say Makudonarudo "correctly."
To answer the broader question, though, humans are born being able to understand and form every possible phoneme that people can use to formulate speech, but they quickly learn to specialize in the phonemes of the local language around them. Brain plasticity decreases the older and older people get, and the more and more stuck you become with a limited set of phonemes at your disposal (unless you really really work at it). Japanese in particular has a very limited set of sounds and sound combinations that are possible, and it completely lacks several English consonants, like L, Q, and V, and it only has context-specific use of consonants like F or H. It's like trying to paint with a very limited palette. It's literally physically difficult for a Japanese person who wasn't immersed in the English language from a very early age to be able to form the proper sounds with their mouth to articulate the language in the way an American would. The word "parallel" is practically the stuff of nightmares.
English, on the other hand, is a pretty damn flexible language, and aside perhaps from the rolled R there's really nothing in the phonetics of the Japanese language that falls too far away from the kinds of sounds that are already used in English. Japanese's pronunciation rules are also quite uniform and not at all subject to the kind of random bullshit that English is. "A" is always going to be pronounced "a" in the exact same way every time you see it. It's not going to be pronounced one way in "father" and a different way in "plane" and yet another way in "cat." That makes it a lot more straightforward to pick up its easily compartmentalized syllabic nature. If you know how the vowels sound and understand the basics of how inflection works, you can say literally every single Japanese word in existence correctly just from being shown it written in text, even if you've never heard it before.
Instead, imagine a similar situation with a different language. I bet you would have a damn hard time saying a whole lot in Xhosa, a language with numerous phonemes that come nowhere close to English. Fuck, you'd probably have a hard time saying the WORD Xhosa (that X is a click, by the way). That's because you'd have to literally train your mouth and tongue to do things it's never done before. It's not impossible, but it's a lot of work. That's a better analogue for how Japanese speakers would approach trying to accurately pronounce English rather than English speakers trying to pronounce Japanese.