Because it is present tense in polite form (존댓말). The second one is the plain form.

1. 어리다 is a dictionary form and also a plain speech form. 2. 어려요 is a present polite ending form of 어리다. 어리다 is a descriptive verb and its stem is '어리'. I put a '|' after the stem. * verb stem + 아요 or 어요 --> present polite ending form 어리| 다 --> 어리+어요 --> 어리어요 --> 어려요 (contracted) ** When the verb stem has 'ㅏ' or 'ㅗ' , '아요' is added. When the verb stem has a vowel other than 'ㅏ' or 'ㅗ', '어요' is added. *** However, there are exceptions to this vowel rule. e.g. 아프|다 (to be sick) --> 아파요 Hope it helps and please follow me at koreanclass.canada on Instagram!