English pronunciation isn't easy
Don't think that, just because you find it easy, most people in the world will; English pronunciation is actually quite complex by any measure. The language has something around 10 vowels (not counting diphthongs) and 44 phonemes; well above the average, and more than double Japanese's 5 vowels and 17 phonemes. What's more, English syllables are unusually complex, and may have long sequences of consonants (as in "lengths") and consonant-only syllables (as in "bottle"). Even people in Spain or Italy will unconsciously add vowels between the consonants in order to simplify English syllables—and Japanese syllables are even simpler.
People generally have trouble with foreign sounds
You say you find it easy to pronounce sounds not in your native language, and I don't doubt you. However, you're being unfair to the Japanese by singling them out; just listen to second-language speakers anywhere in the world, and you'll find that people generally have trouble distinguishing sounds not in their languages. For example, the Chinese speakers I known find it difficult to distinguish sounds like b/d or e/ɛ when learning my native Portuguese. English gringos have trouble with the nasal "ã", or even with saying a simple /o/ without turning it into a diphthong [oʊ]. I have trouble with English t/th; anyone from a non-tonal language has trouble with Chinese tones, and so on and so forth.
Of course, people can learn these sounds (if properly instructed); but it's a well-known fact that many don't, even after years living in another country. This is interesting, because babies always learn all the sounds people speak around them (well, except for a few cases of "speech dysfunction", like lisps; but these are unusual). Adult foreign learners have a lot more difficulty.
English is incorporated into the Japanese sound system
Did you know that around 58% of English words came from Latin and French? But they pronounce them very different than actual Latin or French. The word "Latin" itself, for example, is like [ˈlæ.ʔn̩] rather than /ˈla.tin/. Et cetera is /ˌɛtˈsɛtɹə/ instead of /ɛt ˈkeːtera/, and so on.
My point is, when you incorporate lots of words from a foreign language into your own, you adapt the pronunciation to fit yours. It would be a bother to keep changing from English sounds to Latin sounds all the time.
Japanese has something like 30% "foreign" (gairaigo) words (not counting Chinese), most of which are English. Just like English speakers adapt words like "Et cetera", "Paris" or "Mexico" to fit their own sound system when speaking English, Japanese adapt English words (and others) to fit their own sound system when speaking Japanese. So "hamburger" becomes hambāgā, "cut" becomes katto, etc.
Unfortunately, this means Japanese people have an easy-available but inexact "version" of English already in their minds, and this makes it harder to learn actual English pronunciation. When English people learn Latin, they have to begin by unlearning the way they pronounce Latin words and Latin letters, and get used to actual Latin sounds. Japanese people, too, have to learn to set aside their native gairaigo pronunciation.
If the Japanese person needs to actually speak English, for example to live in England, they will learn it; but if they're just living in Japan and talking to other Japanese people, they'll just use the Japanized English words, for the same reason that English speakers use an Anglicized pronunciation of Latin words when talking to each other.
English classes in Japan usually suck
Japan has mandatory but atrocious English education, which is worse than no education. Richard Schmidt has pointed long ago that foreign learners won't acquire non-native sounds unless you draw conscious attention to them first. However, the typical English course in Japan doesn't try to teach the basics of articulatory phonetics or how to enunciate the sounds; they just keep doing grammar drills and such. I'm living in Germany, and I got a Japanese student to pronounce German-only sounds like "ö" and "ü" in one afternoon by explicitly explaining tongue and lip positions to her. I'm sure that, if a Japanese person is actually taught English phonology, they'll be able to pronounce English without major issues. I've met plenty of Japanese individuals with better English pronunciation than mine (admittedly, they usually have lived overseas, and therefore had incentive to learn actual English pronunciation, as opposed to their native Japanized English).