Here's why:
* Mutual intelligibility: While speakers of both languages can often understand each other to a certain degree, especially in written form, there are significant differences in vocabulary, grammar, and pronunciation that make communication difficult, particularly in spoken language.
* Historical and cultural differences: Ukraine and Russia have distinct histories and cultures, which have shaped the development of their languages.
* Linguistic research: Linguists recognize Ukrainian and Russian as separate languages based on their differences in phonology, morphology, syntax, and lexicon.
It's important to recognize that linguistic identity is complex and nuanced, and the question of whether Ukrainian and Russian are the same language is a matter of ongoing debate. However, the evidence points to them being distinct languages with clear differences.