>>214260114 (OP)
Japanese people are exposed to English from early childhood, through school, media, loanwords, etc. By the time a Japanese decides to actually learn English they have a lifetime of English exposure to base it on, even if they can't speak it the pieces are there in their brain waiting to be put into place
An English person who decides to start learning Japanese has zero knowledge of Japanese except maybe a couple words they picked up from anime. They will have a much harder time gaining exposure to the language. Basically you need to live in Japan for some time to actually become fluent, unless you're a savant at languages