Because хобі is neuter in Ukrainian (as a lot of other uninflectable loan words)
Ukrainian neuter nouns usually end in о (молоко - milk), е (сонце - sun) and sometimes я (these are usually baby animals: слоненя - baby elephant).
і is not a typical ending at all. But хобі is a loanword, so we loaned it just the way it is.
If loanword ends in а/я (typical feminine endings) it usually becomes feminine, if it ends in a consonant (typical masculine ending) it becomes masculine.
If it's ends in о, е or any other vowel - it most likely becomes neuter (журі - jury, табу - taboo). These words are usually uninflectable in Ukrainian (i.e. cannot be declined)
Sometimes loanwords ending in "i" become plural: суші (sushi), жалюзі (window blinds)
Which language is жалюзі from?
Thank you. This is a very helpful explanation! Just what I expected, except that I didn't factor loan words.
It comes from jalousie which is French for "jealousy". English also borrowed it for somehat similar concept: https://goo.gl/rcbahE. They say that these blinds may have got their name (in some Slavic languages) due to the fact that in the East jealous husbands used them to hide their beautiful wives