It's not an opinion - it's a fact... ...and this is the reason. The languages are similar in origin, but one can't say with any sort of integrity that they're similar nowadays. I had a Belarussian friend (her language being one of the three Eastern Slavic languages, the other two being Russian and Ukrainian) speak to me in her native language (despite the fact that we both spoke perfect Russian), and it's indistinguishable to me who's never learned Belarussian. I can discern the written part of both languages, but me being correct on what I think it says is no better than chance. They do have similar roots and one can do fairly good figuring out the written speech, given linguistic intuition and fair knowledge of the roots themselves, but this doesn't make them any closer to each other.(which is bs in my opinion)
growing up in Ukraine, you cannot avoid being exposed to the Russian language on TV, around you etc... which you can't say for Ukrainian language in Russia.